The Complete Guide to Understanding (and Avoiding) Bounce Back Emails

Written by Franco Varriano.

Have you ever sent an email campaign, only to receive a message that some members of your list couldn’t receive it? And this after you put all that hard work into crafting the perfect message. Disappointing doesn’t start to describe the feeling.

The technical term for a message coming back because it couldn’t be delivered to a customer’s email is a bounce back. In this article, we’ll explore everything you need to know about bounce backs, including what causes them and how to avoid them during your email marketing campaigns.

What Is Bounce Back?

How bounce backs show up in the Rare Campaign Reports.

When you try to send an email and it can’t be delivered, you’ll receive a message in response known as a bounce back. Imagine the email is a rubber ball that you’re attempting to throw through a hole. If something obstructs the hole, then the ball will bounce back to you, the sender. Hence the bouncing analogy.

But this is just an analogy. What’s really happening is that your email is passing through a series of mail transfer agents. Mail transfer agents (MTAs) are software that ensures the proper delivery and handling of email messages. When an MTA finds a message that can’t be delivered, it will “bounce” it back to the sender with an error message (we’ll explore the different errors below).

Bounced emails fall into two categories: soft bounces and hard bounces. Let’s examine each in more detail.

Soft bounces are failures in delivery with a temporary cause. There are several possible causes that we’ll explore in a moment, but for now, all you need to know is that soft bounces indicate an issue on the part of the recipient (that will likely be resolved) or an issue you can resolve (your email is too large, for example).

Hard bounces, on the other hand, indicate a more serious issue. When an email hard bounces, it means there’s a permanent issue preventing delivery. It could be that the recipient email address or email server no longer exists, or that they’ve blocked incoming messages from your sender address. There’s not much you can do about hard bounces, but it’s important to recognize them so you don’t waste resources sending to addresses that cannot receive your emails.

Now that we’ve covered the basics of email bounces, let’s examine some common causes of bounced email in more detail.

8 Common Causes of Bounce Email

There are a variety of reasons that your emails may bounce, but most of them are so rare that you don’t need to worry about them. In this section, we’ll cover a few of the most common reasons your emails bounce. For a full list of all possible errors (and their corresponding error codes), refer to the official list of The Internet Society.

1. Invalid Email

One possible cause of a bounce is that you attempted to send an email to an address that does not exist. This is rather unlikely if a user opted onto your email list, but it could happen if you’re manually entering addresses from an old email list (we recommend that you export them as a group instead of entering them individually).

Here’s an illustration of this kind of bounce on a small scale, in which we attempted to send an email to the fictitious address (using Google Inbox).

2. Invalid Domain

Another possible reason for a bounce is that the domain name of your recipient doesn’t exist. This is generally unlikely, since most customers use large providers such as Gmail or Outlook.

But it is possible that someone with their own domain-specific email address could subscribe to your list, later take down (or lose possession) the domain, and never unsubscribe. In this case, email you attempt to deliver to any address at this domain will bounce. If this happens, then most email clients will automatically remove the address from your list, so it’s not something you need to worry about fixing manually.

3. Undeliverable Email

If an email is undeliverable, it means the server to which you are attempting to send the email is unavailable. The most common reasons for server unavailability are that the server has been overloaded or is currently undergoing maintenance.

If this happens, then there isn’t much you can do about it. If it only happens once, it’s not a big deal; future messages will probably make their way through without any bounces. But if it keeps happening, then you should consider whether the email address in question is worth keeping on your list.

4. Email Size

If the email you’re sending is too large, it can cause issues with deliverability. This is a fairly common issue for emails that use lots of images or HTML. The best way to avoid it is to make sure that any images you use in your emails are sufficiently compressed. That is, make sure to use JPEGs and not PNGs.

Generally, your email marketing software will alert you if an email is too large to be delivered before you send it.

5. Full Mailbox

If the recipient’s mailbox is full, then they won’t be able to receive any emails. There’s nothing you can do about this, except hope that the recipient empties their mailbox. This error isn’t as common as it used to be, since the amount of storage available has increased, even with free email accounts.

For instance, a typical Gmail account has a storage limit of 15 GB. Given that the average email is 75 KB in size, a user would need 200,000 emails to fill up their inbox. Of course, in practice the size can vary dramatically (including images or other HTML elements will make them larger), but it still gives a good idea of how unlikely it is for your email to bounce due to a full mailbox.

6. Blacklisting

If your sending domain is on a common blacklist, then it’s possible that it won’t be delivered. A blacklist is a list of domains associated with illegal or malicious behavior such as spam or viruses. They help spam filters figure out which emails to catch.

Even if you’ve done nothing wrong, it’s still possible to end up on a blacklist. How? If your domain is part of a certain cluster of domain names is associated with questionable activity, then it may end up blacklisted by association. Blacklisting of this sort is usually temporary, but if it becomes a persistent issue then you or your webmaster will need to investigate further.

Another possible reason for blacklisting is a greater cause for concern: your site has been hacked and the hackers are using your domain to send spam or other malicious email. This will require more involved action that is beyond the scope of the article. Just know that undelivered emails due to blacklisting could be a warning sign of a site hack.

So how can you know if you’ve been blacklisted? The quickest way is to type your site’s domain or IP address into MXToolbox’s Blacklist Check tool. It searches for your site on a variety of blacklists and then alerts you.

Here’s an example search we performed using

What do you do if you find out you are on a blacklist? Not all blacklists are created equal, don’t worry. For a list of blacklists that do matter (and steps to get your site removed from them), consult this article from rackAID.

7. Spam Filter

Even if you’re not on a blacklist, it’s still possible for your emails to end up in spam if they appear spammy. When you’re using your emails to sell products, this is unavoidable to some degree.

Some spam filters tend to be more zealous than others (looking at you, Gmail), but if your emails are ending up in many spam boxes repeatedly, then it can hurt your sender reputation and diminish the effectiveness of your email marketing.

8. Blocked Email

Finally, it’s possible that your email will bounce because the recipient has blocked incoming emails your address. It’s more likely that a recipient who doesn’t want your emails will simply unsubscribe, but if you’re being too aggressive or frequent with your messages, they might just block you. As long as you follow email marketing best practices, it’s unlikely you’ll get blocked.

3 Ways to Avoid Email Bounce

We’ve already hinted at some ways to keep your emails from bouncing, but here are a few more practices you can follow to ensure that your customers get your emails every time:

1. Use the Right Email Marketing Software

The right email marketing software will help you avoid most of the issues we’ve discussed in this article without any additional effort on your part. You can compare your options here, but whatever you do, don’t use Gmail or another consumer mail client for your email marketing.

This is hugely inefficient, but also can cause trouble when you send bulk messages, since common senders such as Gmail and Outlook impose limits on the number of messages you can send.

2. Don’t Use Spammy Subject Lines

Nothing will land your emails in spam (and ultimately on blacklists) more quickly than spammy subject lines. Avoid LOTS OF ALL CAPS or tons of !!!!!. Don’t misspell words (unless you’re doing so for a particular effect). In general, follow email subject line best practices, and you’ll have nothing to worry about.

3. Send Email Regularly (But Not Too Regularly)

If you haven’t sent an email to certain members of a list in a while, they may get confused and mark your message as spam. So make sure you send messages regularly, once a week at least (as long as it makes sense to do so).

On the other hand, sending too often can also get you blacklisted. The frequency with which you need to send emails to get blacklisted is so high that it’s unlikely you’ll reach it in the course of normal ecommerce marketing, but it’s still something to be aware of.

Start Your Journey to Making More Sales

We hope you now have an understanding of how to avoid email bounce, making sure all those painstakingly composed marketing messages get to your customers.

Of course, to get the messages to your customers to begin with, you’ll need the right email marketing software. For ecommerce, there’s no better option than Rare. Rare makes abandoned cart emails, automatic product recommendations, and other common ecommerce email marketing tasks a breeze. 

And if you’re one of the thousands of merchants who are already using Rare’s Smart Email Marketing software and want to see how our Customer Success Team can work with your brand to grow your revenue – feel free to book a call at your convenience here.

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