Inboxes are crammed with daily emails. The average working person receives about 120 email messages per day.

For companies like yours, standing out in a crowded inbox is more difficult than ever. That is if you make it to the inbox in the first place. Hitting "Send" on your email doesn’t guarantee it. Even "delivered" doesn’t mean "delivered to the inbox."

So, where do they go?

Every day, 31 billion emails bounce. And about 16% of emails end up in the spam folder. While 75% of people typically check their spam folder, you still might miss out on a quarter of your list.

So, what should I do?

This comprehensive email deliverability guide is jam-packed with step-by-step instructions and tips to help your emails stay out of spam. Our email marketing experts share their years of experience and strategies that have helped hundreds of thousands of companies land in the inbox.

Ready to begin? Let’s start simple with the basics.

What is email deliverability?

Email deliverability is a term used to describe your ability to reach your subscribers' inboxes when sending email messages. Strong email deliverability guarantees that your campaigns are reaching your customers so that you can build better email marketing ROI.

However, many senders confuse email deliverability with email delivery. They don’t mean the same thing.

Email delivery is the number of emails that manage to reach a mailbox. However, that could be in the inbox or in the spam folder.

Email deliverability refers to the number of emails that reach recipients’ inboxes. Your email service provider (ESP) will report how many of your emails were delivered, but that doesn’t mean those emails landed in the inbox.

Knowing the difference will help you better understand your email marketing metrics and how email deliverability reflects your overall performance.

What affects email deliverability?

The following factors can significantly impact your company’s email deliverability:

What impacts email deliverability?

1

Email bounce rate

2

Email engagement (opens, clicks, etc.)

3

Email authentication records (DMARC, SPF, DKIM)

4

Email content (spam words, file weight, etc.)

5

Email sender score/sender reputation

6

Your email service provider (ESP)

All of these ultimately contribute to your company’s email domain reputation. Savvy email marketers work tirelessly to build and maintain a positive sending reputation, which can make or break your company’s email marketing results.

What is email sender score?

Email sender score, also called sender reputation, is a numerical score between 0 and 100 that reflects your email behavioral habits as a sender. Much like your word-of-mouth professional reputation, your sender score lets internet service providers (ISPs) know if your company is trustworthy.

For your email deliverability, the question is: do you have a strong sender reputation? If so, your emails are more likely to reach the inbox than those with low sender scores.

Conversely, your email deliverability suffers if your behavior makes you look like a spammer. Sending too many emails, racking up spam complaints, or breaking privacy laws will likely land you in the spam folder. Additionally, many ISPs will block your content altogether.

Mailbox providers handle billions of emails every day. Their goal is to provide relevant user experiences. So, by using email sender scores and sender reputation, they can quickly differentiate legitimate senders like you from spammers.

Email deliverability tip

Your bounce rate has a significant impact on your email score. When more than 2% of your emails bounce, your email score takes a hit. Make sure you check your contacts before you send again.

Email deliverability guidelines to reach the inbox

Now that you know what impacts your email deliverability, it’s time to implement email marketing best practices to bolster yours. We broke down each tactic for improving your sender reputation into categories so that you can successfully get past spam filters and conquer the inbox.

4 essential rules to boost inbox placement

Rule 1 - Separate the types of emails you send

If your company sends transactional emails (receipts, sales reminders, etc.) as well as newsletters, use a dedicated IP address and email domain for each email type.

For your typical corporate transactional emails, use a domain like "your_company_name.com." For all other emails, use a different domain that still resembles your main one, i.e., "your_company_name_news.com."

Each domain should send emails on separate, dedicated IP addresses. This is because when different email types are mixed, Google and other ISPs will more commonly place your content under the “Promotional” or "Other" tabs instead of the inbox.

Rule 2 - Use reCAPTCHA on your registration forms

Google’s reCAPTCHA’s role is to prevent automated bots from registering on your site or subscribing to your newsletters. Sending emails to bot registration or to people who exist (but have been registered by a bot) can hurt your email marketing metrics.

For example, if you email 100 bot registrations to an ISP and none are opened or clicked, it tells the ISP that your emails are irrelevant to the recipients. As this happens, your email deliverability decreases, and you’re more likely to find the spam folder.

Bots can also sign up real people. Emailing people who didn't confirm to receive emails will hurt your inbox delivery rate.

Rule 3 - Use double opt-in confirmation on all emails

Double opt-in confirmation requires users to confirm their registration via a follow-up email. Your ESP provides convenient options for setting up a double opt-in. Here’s how it works:

When someone registers on your site or subscribes to the newsletter, they receive an automated email. The subscriber opens the email and clicks on a link that confirms their desire to receive future marketing emails and promotions from your company.

4 Tips to Boost Inbox Placement

1

Separate the types of emails you send

2

Use reCAPTCHA

3

Use double opt-in registration

4

Use a real-time email verification API

Rule 4 - Use a real-time email verification API

A real-time email verification API is essential for proactively blocking low-quality email data from entering your mailing list.

You can install a real-time email validation API on your website, landing pages, and wherever you collect email addresses. When a user misspells their email address or attempts to use fake contact data, the email verifier API detects and blocks it. A great email verifier can even spot tricky high-risk emails like disposable emails or abuse emails.

Learn more - see ZeroBounce's real-time API documentation

Email authentication (and how to use it)

Email authentication is one or more techniques used to determine if a sender is authorized to send emails on behalf of an email domain.

With how sophisticated spam and spoofing tactics are today, companies must protect themselves with authentication. There’s a chance that your email deliverability may be negatively affected by imposters without your knowledge.

Here’s how to get started.

Use SPF authentication

Sender Policy Framework (SPF) is an email authentication protocol that states whether or not a specific IP is authorized to send emails on behalf of your domain.

You can create an SPF record by using a free SPF generator. Once you have your record, you can add it to your email domain’s DNS entries.

Here are some excellent resources to guide you through setting up and testing your SPF record:

• Guide to mail server configuration
• The SPF Project
• Abusix
• Mail server tester

Use DKIM authentication

DomainKeys Identified Mail (DKIM) is an encrypted key on your emails that mail servers use to determine the sender's legitimacy. The TL;DR - DKIM verifies if the sender is authorized to send your company emails.

First up, here are some excellent resources regarding DKIM:

• Guide to mail server configuration
• The DKIM Project

These DKIM tools will sign your emails. However, most email servers have this feature built-in; you just need to enable it.

• Exchange DKIM Signer
• Limilabs Mail.DLL Component
• Email Architect DKIM for Exchange and IIS SMTP Service

Use DMARC authentication

Domain-based Message Authentication, Reporting, and Conformance (DMARC) is an effective email authentication method that prevents phishing attacks from imposters and reduces spam. A DMARC record enforces a policy that tells the ISP what to do with incoming emails that fail SPF or DKIM checks.

Here are a few helpful DMARC resources:

• Guide to mail server configuration
• The DMARC Project

And, here you can create a free DMARC record:

• ZeroBounce DMARC Generator

Use a reputable DNS provider for optimal email deliverability

Successful mail delivery is dependent upon being able to retrieve DNS records reliably. A high-quality, trustworthy DNS provider ensures that this occurs successfully.

ISPs will also check the reputation of your nameservers. Here are some notable DNS providers:

• DYN Managed DNS - Provides a lot of tools and options to secure your DNS properly
• Cloudflare - Provides DNS management and a suite of optimization tools

Note - DNS from large registrars is generally okay, but we have detected slowness from some when applying DNS changes.

Email deliverability tip

Use a trustworthy, reputable DNS provider to ensure senders can access your DNS records. Successful delivery depends on it.

Use a reputable CDN for static content

A content delivery network (CDN) caches different versions of your content worldwide for faster loading and easier viewing.

There are four primary reasons for using a CDN:

• To provide readers with faster load times, especially for image or video content
• ISPs won’t block CDNs as they’re widely known and reputable
• CDNs provide DDOS (Distributed Denial of Service) protection, which keeps your website online even if it’s attacked
• You will benefit from advanced firewall protection that mitigates many classic attacks of which your site would be otherwise vulnerable to

CDN providers:

• Cloudflare (Note - ZeroBounce is a registered partner of CloudFlare)
• Microsoft Azure CDN
• Amazon CloudFront

Example of a DNS record for CDNs

DNS Example
A Record (1.2.3.4) > "Points To " > PTR Record > "Points to" > Hostname (example.com)
(Forward)
Hostname (example.com) > "Points To" > PTR Record > "Points to" > A Record (1.2.3.4)
(Reverse)

Email deliverability tip

ISPs won’t block content from CDNs since they’re trustworthy and reputable. They also provide a better user experience for your email readers.

Use a DNS point record (PTR records) for sending IPs

This email deliverability tactic is mandatory. Most ISPs require Forward Confirmed Reverse DNS (FCrDNS) or reverse DNS lookups. It all sounds like complicated jargon, but it’s quite simple.

Here are some vital resources on FCrDNS:

• ZeroBounce - PTR records
• SPFBL - Introduction to FCrDNS

And a few places to test FCrDNS:

• ZeroBounce mail server tester
• Junk Email Filter FCrDNS Test

Use email warmup tools

Warming up an email IP address is critical. Gradually introducing your emails to a new IP allows your company to establish its sender reputation. Once you have a foothold, you can reap the benefits of your email marketing efforts.

However, don’t send too many emails at once. If your sending platform supports the feature, set limits per hour or day for each of the IPs per email domain. If you send too many emails early on, you’re more likely to see your bounce rate increase.

Here are the suggested limits per provider that you should follow in your first 30 days of mailing:

Suggested rate limits for email service providers

200 emails/day/IP (for at least five days, then you can double-up every day)

200 emails/day//IP (for at least five days, then you can double-up every day)

200 emails/day/IP (for at least five days, then you can double-up every day)

200 emails/day//IP (for at least five days, then you can double-up every day)

50 emails/day/IP

100 emails/hour/IP

100 emails/connection/IP (up to 5 IPs)

Zero Bounce

You should also always visit the postmaster website of the domains you’re trying to send to in bulk. You’ll often find the sending limits published within their bulk sender guidelines.

Here are some example strategies for email warmup:

• Start a new warmup for the existing IP
• Send only to subscribers who have opened at least one email in the last 30 days
• Limit your starting volume to 3,000 subscribers
• Keep sending to those subscribers only for the first three days before increasing the volume
• Increase the volume by 1,500 subscribers with opens in the last 30 days. Your new total volume should be 4,500 subscribers
• Keep sending to the new volume of 4,500 for two more days
• Follow this strategy to increase the volume every two or three days by 50% of the actual volume
• After ten days, you can start increasing the volume by doubling it

Check your feedback loops (FBL)

Feedback loops (FBL), also called complaint feedback loops, is a service that alerts email senders to spam complaints. Whenever a subscriber marks your email as spam, you’ll receive a notification from the ISP. This service aims to help you clean your list of uninterested subscribers while helping the subscriber keep their inbox free of unwanted mail.

Email deliverability tip

Feedback loops are provided by some ISPs to aid senders in keeping a clean list and preventing the subscriber from getting unwanted mail

A list of feedback loops can be found below:

• AOL1
• Comcast
• Earthlink - send an email to : fblrequest@abuse.earthlink.net
• Excite
• FastMail
• Gmail
• Hotmail
• Italia Online
• Mail.ru
• RackSpace
• Synacor
• Mail.ru
• Terra.com.br
• Tucows
• Yahoo1
• Zoho

Gmail has a feedback loop that’s only available for ESPs who are MAAWG members (Messaging, Malware, and Mobile Anti-Abuse Group) and are approved by Google as trustworthy senders.

Major companies often use the same blacklist providers. Being listed on a single blacklist can affect your delivery to many different ISPs.

Delivery monitors and IP/domain health monitors

ISPs often provide tools to monitor your IP addresses, domain health, and email delivery.

However, you can also sign up with these trusted services to actively monitor your email domain health:

Google Post Master

• Spam rate
• IP reputation
• Domain reputation
• Feedback loop identifiers
• Authentication rates for DKIM, SPF, and DMARC
• Encryption rates (if you send emails using TLS)
• Delivery errors

Hotmail

• IP reputation
• Complaints per IP
• Traps per IP and the date
• Ehlo/Helo commands

Email blacklist monitoring and how it impacts email deliverability

Landing your email domain on a blacklist can devastate your email marketing. A single blacklisting will affect your ability to deliver to countless ISPs.

You can detect any unusual activity and get alerted of a potential blacklisting for your domain with blacklist monitoring tools.

What’s an email blacklist?

An email blacklist is a collection of known email domains and IPs recognized as untrustworthy senders. ISPs use blacklist service providers to monitor which emails to block to protect their users.

What are the types of email blacklists?

There are three primary types of email blacklists:

Three primary types of email blacklists

Public

These blacklists are published so that any ISP can use them. They are the easiest ones to monitor using automated tools.

Private

These are paid email blacklists. The only way to actively monitor these is to use inbox testing tools and check your delivery.

Internal

These are maintained by the ISP directly. They can also be monitored with inbox testing tools.

Some ISPs use multiple email blacklists, so you must monitor all aspects of your sending using the tools available.

Email deliverability tip

A single blacklisting will tank your email deliverability. ISPs automatically block your emails, which further compounds your deliverability issues.

Here are a few places you can check your IPs that are not included in the automated blacklist checks mentioned above:

• Barracuda Central
• Sophos Threat Center
• Mcafee Threat Intelligence (now Trellix)
• Symantec IP Reputation
• Cloudmark IP Remediation Portal
• Proofpoint IP Reputation Lookup
• Trend Micro IP Lookup
• GoDaddy's Secure Server
• Hetzner Online
• Invaluement
• Manitu
• Linux Magic
• SURBL Blacklist
• HRBL Blacklist
• FortiGuard
• Cyren IP Reputation
• (Cisco) Talos Reputation
• ICORP
• WIFI4INDIA Blacklist Lookup
• Clean MX
• Death 2 Spam
• SonicWall
• Postmaster.Free.FR

How to delist yourself from blacklists in non-obvious places:

• CenturyLink- [C10] RBL restriction: Blacklisted by Internal Reputation Service - Send an email to "postmaster [at] embarqmail.com" with the subject line "Please investigate for spam-like qualities" and list your IPs
• Hotmail- Fill out this form
• United Online (Netzero/Juno/Bluelight)- Fill out this form
• Vade Secure- Fill out this form

Email whitelists can guarantee email deliverability

An email whitelist lists email addresses, IPs, or domains approved to send emails. A whitelist is effectively the opposite of an email blacklist. Whitelisting your IPs provides many benefits at different ISPs:

• Increased inbox placement rates for better email deliverability
• Increased sending volumes
• Less restrictive spam filters

Whitelist your IPs after 90 days of sending

It’s recommended to wait 90 days as some whitelisting services will need to see your sending history before you can use them.

Free whitelisting services:

• United
• DNS WL - Helps with places that use SpamAssassin.
• inbox.lv

Paid whitelisting services:

• Return-Path - This is one of the best whitelist programs.
• CSA (Certified Senders Alliance) - Based out of the EU.
• ISIPP - Helps with places that use SpamAssassin.

Email blacklist

Versus

Email whitelist

Marks your domain or IP as unsafe and untrustworthy

Automatic email rejection

Higher bounce rates

Lower sender reputation

Lower email engagement

Marks your domain or IP as a recognized, trustworthy sender

Allows all incoming emails

Allows for increased sending volume

Less restrictive spam filters

Better open and click rates

Improve your email list quality; check email addresses regularly

The quality of your email list is one of the most critical factors for email deliverability. Your email deliverability will improve if your email list contains high-quality, valid, engaged subscribers. On the other hand, repeatedly bouncing emails due to invalid email addresses or emailing spam traps will lower it.

You can enhance your email list quality quickly with email verification. Even if you’ve built your email list carefully, it’s wise to refresh it with clean emails every few months, as your emails decay by almost 26% annually.

Email deliverability tip

Email lists decay by nearly 26% annually. People leave jobs, get promoted, or simply create a new email address. Use email verification to keep your list up-to-date.

Remove risky email addresses from your list

Cleaning your list with email verification will also detect risky email addresses such as disposable emails, spam traps, and abuse emails. After identifying them, remove them and reupload your clean email list to your mailing service.

Eliminating these emails from your list will improve your sender score, domain and IP reputation, and email deliverability.

Monitor your sender score

Domains use your email sender score to determine whether or not to allow emails from specific IP addresses. If your sender score is less than 90, it can indicate that you have poor email list quality. Upload your list for email verification, check your recent campaign results, and closely.

A lot of domains use sender score as a determining factor to allow emails from your IPs. If your sender score is less than 90, you should take action to improve it.

Verify your email server is set up correctly for sending emails

Your email server configuration plays an essential role in your email deliverability. That’s why it pays to check your email server settings before sending your next campaign. To do this accurately - use a mail server tester.

With a ZeroBounce email deliverability toolkit, you’ll get a detailed technical report of your email server configuration. This report highlights any issues so your team can investigate and remedy them before publishing your campaign.

Create abuse@ and postmaster@ email addresses for your sending domain

Easily overlooked, ISPs rely on these two email addresses for feedback loops and complaints. Not configuring these two accounts is a red flag for ISPS, as RFC standards also require them.

Double-check your email content

Be sure to use the recipient’s name in the subject line and the header of the HTML (e.g., “Dear John”).

Also, check for spam words and domains in the HTML. For example, if you accidentally link to a blacklisted domain, your email will likely bounce. An inbox placement tester can help you catch these issues before you risk more email bounces.

Additional tips to improve email list quality

• Avoid buying or renting an email list - ESPs can detect if a list is purchased, even if it’s from a reputable seller. They can and will reject it when attempting to import it to your mailing platform. Email validation companies won’t be able to assist you, either.
• Check open and click rates - Aim for an open rate of at least 20% and a clickthrough rate of at least 1%.
• Remove non-opener emails after six months - If the subscriber isn’t opening your emails, there’s a chance the address is valid but no longer in use. Either way, the non-engagement brings your email deliverability down.

Email deliverability tip

Email list quality significantly impacts email sender score. If your sender score is less than 90, take action ASAP to improve it.

Industry best practices for high email deliverability

You’re well on your way to becoming an email deliverability expert. But if you want to email like the top email marketers, check out these 15 best practices to guarantee the highest email deliverability possible.

Remove inactive subscribers

Remove subscribers who don’t open or click your emails after 180 days. A lot of ISP algorithms are based on user engagement. So, the more non-openers and non-clickers you have on your list, the worse this metric gets and the more your email deliverability plummets.

If you wish, you can segment these emails and attempt to re-engage them after six months to see if the recipient opens or clicks.

Ensure you don’t use too many connections per IP

Every ISP limits the number of connections per IP address. Always review the Postmaster Guidelines of each ISP regarding Rate Limiting.

Here are some safe values to use for your connections:

Retry sending when receiving temporary error codes

One of the most common ways that ISPs reduce spam is a technique called “greylisting.” This anti-spam technique will create a temporary error indicated as SMTP 451. When this occurs, the ISP expects your mail server to be configured to reattempt email delivery at a later time. This is standard; your configuration must be correct to improve your email deliverability.

When attempting to resend the email, you must use the same IP. Rotating the IP will just result in additional greylisting.

Retry times should be configured using the schedule below:

How often should you reattempt email delivery?

Do this when you encounter greylisting, full mailboxes, and other temporary errors

1

First retry -

15 minutes

2

Second retry -

45 minutes

3

Third retry -

2 hours

4

Fourth retry -

6 hours

5

Fifth retry -

12 hours

This will also help if you have unknown email server configuration issues, so you won’t accidentally attempt too many retries. Attempting to retry too many times consecutively can negatively affect your email deliverability and sender score.

Monitor your brand for compliance

Email privacy and compliance rules are mandatory and non-negotiable. You must honor laws such as , CAN-SPAM, CCPA and EU GDPR to send emails to users in the applicable locations. If you don’t have a person’s express permission, it will negatively impact your email deliverability and present legal consequences.

Review the company's security and compliance guidelines when using any email service to ensure you take advantage of the highest security and privacy standards possible.

Use a Reply-To header that’s valid

We’ve all received emails from noreply@, but having a functional Reply-To email address is far better. Actively monitor your Reply-To address and respond to all customer inquiries. The more engagement your ISP sees between you and your customers, the better your email deliverability.

Send only one email per connection

You should only send emails to one person at a time. You’ve seen those emails with many random “CC” or “TO” recipients - it’s a colossal spam flag.

When sending bulk emails, ensure each email is addressed to a single recipient instead of multiple contacts simultaneously.

Enable outgoing TLS connections

Mail servers are designed to be opportunistic. They prefer to use the most secure protocols possible when accepting mail in this order: TLS 1.2, TLS 1.1, TLS 1.0, and no TLS.

Transport Layer Security (TLS) is a standard for secure email, which ensures that communications remain strictly between sender and receiver.

Some mail servers will refuse all non-TLS transmissions. So, to achieve maximum email deliverability, enable TLS when sending content.

Keep abuse/complaint rates low

After taking our advice above to sign up for feedback loops, you must follow up by actively removing known complainers from your email list. Once you receive the alert from your FBL, find the contacts in your list and delete them.

Guidelines for acceptable spam complaint and bounce rates

Spam complaint rate -

Below 0.1% (1 in 1,000 emails sent)

Bounce rate -

2% or lower

Keep your bounces and spam complaints low for optimal email deliverability

When spam complaints exceed 0.1% (1 in 1,000 emails), it hurts your email deliverability.

Email deliverability tip

Spam complaint rates will destroy your email deliverability. Keep your spam complaint rate below 0.1% and eliminate known spam complainers.

Don’t use private WHOIS for your email domains

All registered email domains must have accurate information by ICANN. That includes an accurate WHOIS response, which lets internet users know who owns the email domain.

Hiding behind a private WHOIS can hurt your domain reputation and, in many cases, is illegal.

Maintain a privacy policy on your website

Whenever you ask an ISP to whitelist your domain or IP, they look for your privacy statement to verify its legitimacy. Ensure your privacy policy remains up-to-date and offers complete information regarding your security and compliance.

Use DNSSEC

Domain Name System Security Extensions (DNSSEC) is a technology that prevents the hijacking of DNS lookups. Using DNSSEC is not a requirement but a highly recommended security feature that can help legitimize your domain and improve email deliverability.

Remain consistent in your sending behavior

Send your newsletters, promotions, and other marketing materials on the same days every week and month. Maintaining a consistent schedule proves to ISPs that you have a legitimate business and keeps your IPs warm. The more predictable your sending behavior, the less you look like a typical spammer - and the better your email deliverability.

Don’t segment emails per destination ISP

Separating your emails based on the destination, unless under exceptional circumstances, has a detrimental effect. Some domains will use various services for reputation lookups, but those IPs will never get their sender score increased because they don’t report back to Return-Path.

Don’t send affiliate email marketing

Most ISPs blacklist the subject lines and content for affiliate emails. Remember, everyone is sending the same affiliate promotion. ISPs have AI that learns subject links and content. Your IPs will get blacklisted or will be limited to only being able to send to the spam folder. Typically, ISPs will accept and won’t complain if you add an advertisement to your newsletter.

Email privacy laws and compliance

Honor all unsubscribe requests (CAN-SPAM)

You can’t keep subscribers on your email list against their will. Plus, it’s a terrible way to boost your email deliverability.

When a reader asks to unsubscribe - whether through an unsubscribe link or manually - honor their request promptly. CAN-SPAM law specifies that you only have ten days to do this.

Email deliverability tip

When you comply with email law, everything becomes easier for you to whitelist and improve your email deliverability.

Comply with all privacy laws

It’s critical to stay up-to-date with the latest privacy laws in email. Be sure to comply with CASL and CCPA for CA, CAN-SPAM in the United States, DPEC, and GDPR for the European Union.

When you adhere to the law, everything becomes easier to whitelist, and managing your reputation is obstacle-free.

Here are resources to learn more about significant email privacy laws:

• CAN-SPAM (US)
• CASL (CA)
• DPEC (EU)
• GDPR (EU)
• Digital Personal Data Protection (IN)
• HIPAA (US)

Establishing MX records

A mail exchanger (MX) record specifies which email server should accept incoming email messages on behalf of the destination domain. The Request for Comments (RFC) standards state that if the mail exchange (MX) record is missing, one should use the A record as the mail server.

Email deliverability tip

• MX record - Mail exchanger record; specifies which email server should accept email on behalf of the domain
• A record - A record that determines which IP address hosts the email domain

However, many ISPs don’t follow this standard, and they check the validity of your MX record before allowing any emails from your domain.

If you’re like for an ideal mail service provider for your domain, we recommend using Google’s G-Suite.

Test your MX records before sending

Different ISPs use different spam folders. What Yahoo considers inbox-worthy, Gmail may put into spam. Whether you’re testing content, images, or mail configuration like MX records, it’s vital to perform inbox testing to protect your sender reputation and email deliverability.