What to do when your email bounces

Email bounces – that pesky phenomenon that happens when email messages cannot be delivered to a recipient’s email address. They seem to persist with each new email you send out. Here’s what to do about it.



There are a variety of reasons for an email bouncing, and we’ll touch on the main ones in a moment. When it happens, a "return to sender" message (also known as an SMTP Reply) is applied and sent from the recipient's email server with a more detailed explanation for the bounce back. It's an annoying reality in email delivery, because you just want your emails to get delivered to all your subscribers.

Why are high bounce rates such a hard problem to solve? This is largely due to the fact that, in some cases, bounced email addresses are really technical errors that can only be remedied with some due diligence. Still, with a quick fix, your bounces should go down, right?

Well, that depends on what type of email bounces you’re getting.

What are email bounces?

Email bounces (which make up part of your total delivery) occur when an email cannot be delivered to a recipient’s inbox. You will usually receive an auto-reply to your message that gives you a reason for the non-delivery. There are two types of bounces you can run into when sending email – hard bounces and soft bounces.

What are hard bounces?

Hard bounces, also known as permanent failures, mean the message is undeliverable due to some sort of unchangeable, permanent reason. Common reasons for hard bounces include:

  • Invalid email addresses such as canceled accounts

  • A receiving server that no longer exists

  • Misspelled domain names

  • Misspelled recipient addresses

Most email providers – Mailgun included – will stop attempting to send hard bounce messages after the first attempt. If it has nowhere to go, not even we can deliver it.

A common misconception is that nothing you do will change this type of bounce, so some senders choose to just throw those email addresses off their lists or CRM. But deleting customer data is never recommended. Instead, emails that hard bounce should be added to a suppression list (Mailgun does this automatically).

Why are suppression lists a better option?

Consider this scenario: One of your recipients changes their ISP from Yahoo to Gmail and forgets to update their email account on your system. Suddenly, you receive a hard bounce from this email address. The customer still wants to hear from you, so deleting their record would mean losing a customer. Instead, we use this hard bounce as a reminder to prompt them to update their valid email address another way, either in-app or using a CPaaS method.

What are soft bounces?

On the other hand, soft bounces (temporary failures) cannot be delivered due to some kind of temporary issue. A few possible causes include:

  • The email message is too large

  • The recipient’s email server is down

  • The recipient is using a vacation auto-reply

  • Their mailbox is full

  • Your email message is blocked for some reason

With so many possible causes, there isn’t any one solution for soft bounces. But the email can’t be delivered to the recipient’s mailbox at that point in time. In these cases, most ESPs (email service provider) will attempt delivery a few more times in case circumstances change and then stop if the email continues to bounce.

Bounced back emails are just one of many possible outcomes when sending an email – read our ultimate guide to email metrics to better understand your email stats.

What do email bounces tell you about your email deliverability?

When emails bounce, they do not get delivered. This means a higher bounce rate equates with a lower email deliverability rate.

You shouldn’t expect to see a sudden increase in hard bounces at any point while sending emails. The only time you might expect a surge is if you are sending to a list for the first time without validating it. Typos and expired email addresses will be present on such a list and will lead to more hard bounces.

Should you keep sending to soft bounces?

Soft bounces are a little different from hard bounces in that there isn’t some one-size-fits-all solution for them. Some soft bounces have easy solutions, like reducing the size of an email that’s too large. For others, it can get a little complicated.

For example, what if the soft bounce is caused by a full mailbox? Some senders might want to continue sending to that inbox, presuming the recipient will eventually clear it out to free up space for more emails.

However, the more likely reality is that the recipient has abandoned that email address. Full inboxes are usually a telltale sign that your email list hasn’t been cleaned in a while. That email address should be treated as invalid and sent to a suppression list.

Sometimes the cause of a soft bounce will get cleared up. But if you notice repeated soft bounces for the same addresses over and over, the smart move is probably to add those subscribers to a suppression list.

You can also send a re-engagement email campaign to all subscribers who have been inactive for a certain length of time, such as a year, and let them know that if they want to remain subscribed they need to respond. You can remove those who don’t respond.

What about a sudden increase in soft bounces?

If you find yourself in this situation, the first task is to look at your list.

We have seen situations where the sender has a clean list, has properly warmed up their dedicated IP address, and has an active subscriber list engaging with their emails – yet somehow, they still end up getting hit with account limitations or soft bounces.

It is possible for the email service provider to make a mistake and misclassify your domain or IP address as risky. In this case, your ESP will have a recovery process in place to solve it.

There is also the chance that you’re getting a higher soft bounce rate because your ESP has placed limits on your account.  We’ll talk more about that a bit later.

How to prevent email bounces

The steps you’ll take to reduce your email bounce rate depends on what’s causing it.

For hard bounces, these are not likely to ever receive emails again. As stated earlier, just add those email addresses to a suppression list and your hard bounces will come down quickly. It’s as easy as that, with (hopefully) no profanity involved.

Avoiding hard bounces in the future and keepings soft bounces down requires a little more effort. Here are a few strategies:

Email verification

For existing email lists, your best strategy for cutting down hard bounces is to use email verification. Some addresses may have typos. Others will be expired or invalid. Others could even have turned into spam traps. Email verification identifies all the undeliverable email addresses on your list, making it easy to add them to a suppression list.

Plus, with real-time email verification, you can continually vet new email subscribers to weed out typos, bots, and other errors that lead to false addresses getting added to your email list. This ensures all your emails are going to the intended recipient.

Double opt-In

In addition to verifying your existing list, you can add a double opt-in feature to your forms to ensure that new subscribers’ emails are correct.

This works because, if a subscriber types an incorrect address into your form, they won’t see the double-opt in autoresponder, and the incorrect email won’t be added to your list. This cuts down on wrong addresses, giving you a cleaner mailing list from the start and protecting your sender reputation.

Remove inactive subscribers

This is another proactive strategy you can employ to cut down on both types of email bounces.

For example, if someone joined your email list using a work email and has since moved on from that job, that email address will become inactive. Eventually, the company will probably de-activate it entirely, which would turn it into a hard bounce. But some companies might dither on that, meaning the account could continue piling up emails until it gets full, leading to a soft bounce.

By removing inactive subscribers once or twice a year, you can stay ahead of situations like this and keep your bounce rate down.

Cleaning your list is especially important if you notice a sudden increase in soft bounces. Perhaps you never cleaned or verified it, or it’s been a while. How old is your list? How were these contacts obtained? When was the last time it was cleaned? Cleaning it up should be your top priority.

Can my ESP place limits on my account?  

Yes, ESPs will limit how much damage you can do to your reputation, and this is reflected in your soft bounces. Before you bring out your torches and pitchforks – hear us out.

Chances are, if you’re experiencing soft bounces in this way, it may not necessarily be a bad thing. Usually, there’s something you’re doing that would adversely affect your reputation. Other times, it could be caused by other email senders if you’re on a shared IP. Allowing risky sending to continue from a shared sending IP hurts other well-meaning senders. So, the ESP takes action so they can protect their other senders.

There are other cases beyond soft bounces where you can find yourself limited while sending.

Sending too many messages at one time without a proper warm up, hitting spam filters with risky sending practices, or sending to recipients who did not consent to your email messages (and thus file spam complaints) are a few additional reasons why an email service provider might limit your account.

Want to know more about email best practices? Check out our Email Growth Playbook, a database of 60+ tactics to help you increase the performance of your email campaigns.

Email bounces can be solved  

We emphasize this a lot when we talk about bounce rates, but we’ll reiterate here – the problem isn’t your bounces, it’s what’s causing them in the first place.

Bounces are almost always a symptom of out-of-date or poor sending practices, and that means poor deliverability. If you're currently trying to do some technical problem-solving with email bounces, we have the documentation for you!

You could have an amazing email marketing campaign, but that means nothing if you aren’t landing in the inbox. Focus on healthy deliverability sending habits and the bounces will take care of themselves, and keep you from being limited by your ESP in the future.

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