Email Delivery Failure: 12 Common Causes (& How to Fix Them)

March 20, 2024
Written by
Jesse Sumrak
Opinions expressed by Twilio contributors are their own

Email Delivery Failure: 12 Common Causes (& How to Fix Them)

You work hard to create emails that engage with your customers or audience, so it can be very frustrating to learn that some recipients never received your email during an email delivery failure.

Despite the fact that your customers request and agree to receive your emails, Internet Service Providers (ISPs—like Gmail, Yahoo, and AOL) and other filters can make many of them go undelivered.

What’s a sender to do?

First of all, don't take it personal. ISPs are just trying to sift through over 100 billion emails that are sent daily.

This volume of spam makes it extremely difficult for ISPs to determine which mail streams are legitimate and which ones are just trying to dupe customers.

Add in bad actors, such as phishers and spoofers, to the mix, and deciphering the email delivery matrix becomes even trickier.

Therefore, it’s important that you follow email best practices so you can help ISPs better identify your mail as legitimate.

Below, we're breaking down the most common causes of email delivery failure—and (more importantly) what you can do about it.

What is email delivery failure?

Email delivery failure refers to when an email message fails to reach its intended recipient or is not successfully delivered to their inbox. Instead of reaching the recipient's mailbox, the email encounters an issue during the delivery process and is returned to the sender or discarded by the email system.

When an email delivery failure occurs, the sender typically receives a notification known as a "bounce-back" or "non-delivery report" (NDR). Reviewing these messages can help pinpoint the exact issue and guide troubleshooting efforts.

Understanding the cause of email delivery failures is crucial for troubleshooting and resolving the issue. It ensures that future email communications are successful and allows you to take the right actions, such as correcting email addresses, adjusting email content, or addressing technical problems with your email systems.

12 causes of email delivery failure

1. Industry sender requirements

Major Internet Service Providers (ISPs) like Google and Yahoo periodically updating their sender requirements. These changes aren't always just minor tweaks—they can significantly affect your email deliverability and, by extension, your ability to connect with your audience.

While these updates aim to shield users from spam and phishing attempts, they also raise the bar for legitimate senders striving to reach their audience's inboxes.

  • Google: Google's updates focus heavily on enhancing security measures against spam and phishing emails. One of the critical areas is the authentication of emails through protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance). Google is now more stringent in analyzing these authentication signals to filter emails more effectively.

  • Yahoo: Similarly, Yahoo has tightened its policies around sender reputation, emphasizing the need for consistent sending volumes and engagement rates. Yahoo is scrutinizing how recipients interact with your emails more closely than ever, using this data to inform its filtering decisions.

2. High complaint rate

A high complaint rate is one of the tops reasons that your email will end up in the spam/junk folder or put your IP on a deny list. Complaints are registered when a user marks your email message as spam (by hitting the “Report Spam” button).

Each ISP has a threshold for spam complaints and if your email campaign crosses the line, your emails will never make it to your user. This is because ISPs are adamant about their customers only receiving wanted email.

You can avoid this problem by signing up for feedback loops. Feedback loops arm you with the email addresses that complained about your email so that you can swiftly remove them from your list.

You can even automate this function like TeamSnap did so you can easily maintain a clean, healthy list devoid of complainers.

3. Email infrastructure issues

Building your own email infrastructure can be complex and expensive, requiring dedicated resources and servers. While many companies opt to maintain their own mail servers, an incorrect configuration can put a halt to your email delivery plans. This is, in part, because spammers tend to focus on sending malicious email, not so much on making sure their email servers are in proper working order.

Also, be sure to follow a few rules of thumb when sending from your own servers. These include sending from a dedicated IP address if you are sending high volumes of email, setting up “postmaster” and “abuse” mailboxes for your domain that you monitor regularly, and ensuring your servers are secure.

Also, avoid open relays or open proxies for maximum email security. Alternatively, you could work directly with an email service provider to avoid the complexities of maintaining your own server.

4. Permission-based problems

Good (or bad) email delivery always starts in the same place—with your email list. A good list is built based on strong permission standards. A strong opt-in strategy will go a long way in building an engaged audience.

However, just because you got permission once doesn’t mean this permission lasts for a lifetime. User needs change over time, and you’ll need to compensate for list fatigue and poor response rates as a result.

Here are some tips and best practices to maintain good email delivery rates:

ISPs now pay close attention to engagement rates, so you need your users to regularly open and click on your emails. Here are some tips and best practices to maintain good email delivery rates:

  • Systematically weed out your non-responders by asking for permission to continue emailing them.
  • Periodically ask your users to update their preferences in your preference center.
  • Be sure your unsubscribe functionality is working properly and easily found too.

In the end, it’s better for users to proactively remove themselves from your list than to go dormant or report you as spam. The latter sends red flags to the ISPs, which can create problems for your email program.

For more tips, check out our free guide on How to Authentically Grow Your Email List.

5. Invalid email addresses

It's impossible to send an email to an address that doesn't exist.

Sending emails to invalid or mistyped email addresses often results in delivery failures. This can happen when recipients provide incorrect email addresses or if there are typos or formatting errors in the email address.

Fortunately, there's a way you can fix and automate this at scale. Try using SendGrid's Email Address Validation API. Leverage the power of our real-time API to identify and eliminate invalid email addresses from your mailing list.

By integrating this API, you can reduce your bounce rate and enhance your relationship with mailbox providers—leading to improved email deliverability.

6. Full mailboxes or storage limits

You won't encounter this problem often, but it's worth noting that some email delivery failures will be caused by your recipients' mailbox capacity. If they've been storing emails and files for years without cleaning up their inbox, your message might exceed its limits.

In this case, the email server may generate a "mailbox full" or "quota exceeded" error, causing the message to bounce back to the sender.

7. Spam filters

Spam filters might mark your legitimate emails as spam and prevent them from reaching your subscribers. This can happen if the email contains certain keywords, suspicious attachments, or if the sender's IP address or domain has a poor sender reputation.

Experiencing this issue? We can help.

Our Email Testing tool is built into the email editor to help you check your messages for potential spam content before pressing send. We'll give you a score to let you know how your email looks measured up against the most powerful spam filters—and we'll also provide recommendations for how you can fix it.

8. Blocklisting

If your IP address or domain is blocklisted by email service providers or anti-spam organizations, your emails may be rejected or filtered out. Blocklisting typically happens when the sender is identified as a source of spam or other abusive email practices—but that doesn't necessarily mean you're a spammer.

It could just be an innocent mistake.

Learn more about blocklists and how to get yourself off of them in our comprehensive how-to piece.

9. Server issues

Temporary server issues, maintenance activities, or unexpected downtime on the sender's or recipient's email servers can cause email delivery failures. During such periods, emails may be queued or delayed, and some may ultimately fail to deliver.

The same can happen with network issues.

Connectivity problems between email servers can disrupt email delivery. If there are communication breakdowns or interruptions, the sending and receiving servers may be unable to establish a successful connection, resulting in delivery failures.

10. Attachment problems

It's best practice not to include attachments with your email marketing messages, but sometimes it'll be necessary. However, some email systems or providers impose size limitations on email attachments.

If the attached file exceeds the allowed size, the email may fail to deliver or bounce back.

11. Sender authentication issues

Email authentication mechanisms like the following play a role in email delivery:

Misconfigured or missing authentication records can cause delivery failures or lead to emails being marked as spam.

12. Recipient email configuration

In some cases, the recipient's email server configuration may be set up in a way that rejects or blocks certain types of emails. This could include specific attachment types, email formats, or policies set by the organization.


These three tips above will help ensure higher delivery rates and reduce the cases of email delivery failure to your email list. You’ll still need to closely monitor your email reputation, but if you focus heavily on avoiding complaints, maintaining a solid email infrastructure, and developing an ongoing permission strategy, you’re winning more than half the battle.

To learn more about email deliverability best practices and how to comply with ISPs, download our Email Deliverability Guide.


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