You’ve spent a bunch of time crafting the perfect, personalized cold email — only for it to bounce.

Frustrating, huh?

But email bounce is a much bigger deal than a single email failing to reach the desired destination. Left unchecked, it can derail your entire outreach strategy.

Scary stuff. But you’re in the right place, because in this article we’re going to explain everything you need to know about email bounce, including:

  • Why emails bounce (and why it matters)

  • What the different email bounce codes mean (in plain English)

  • How to stop your emails from bouncing

Let’s get into it…

What Is Email Bounce?

Let’s start with the basics:

An email bounce is like a “return to sender” stamp on a piece of physical mail. Automatically sent by a mail server or mailbox provider, it tells the email sender that their message didn’t end up in the intended recipient’s inbox.

When one of your emails bounces, you’ll typically receive an error report that includes:

  • The message’s unique ID

  • The date and time of the bounce

  • The IP and hostname of your email service provider

  • The diagnostic bounce code explaining what went wrong (more on this later)

Why Do Emails Bounce?

When you post a letter, there are lots of things that could stop it being delivered successfully. Maybe you wrote down the address wrong; perhaps there are technical issues with your package — like it’s too heavy, or has insufficient postage; or maybe it just got lost on the way.

Similarly, emails can bounce for a bunch of common reasons, including:

  • The recipient’s email address doesn’t exist

  • Their inbox is full

  • An email server outage stopped your message being delivered

  • Your sender reputation is too low

  • Content in your cold email triggered a spam filter

  • Your sending domain has a restrictive DMARC record

To complicate matters somewhat, the term “email bounce” actually refers to two different types of bounces…

Hard vs Soft Email Bounce

There are two main types of email bounces:

  1. Soft bounces: These happen when a temporary issue stops your email from being delivered.

  2. Hard bounces: These occur when a permanent issue is affecting the delivery of your email

As you can likely guess, soft bounces are less serious than hard bounces. But they still shouldn’t be ignored, because after three failed sends, most email service providers will recategorize a soft bounce as a hard one.

Common Bounce Codes and How To Understand Them

Most email clients use something known as the “Simple Mail Transfer Protocol” (SMTP) to push a message from sender to recipient.

Every email sent through this protocol generates an SMTP code. Often, this code will essentially say: “This email was sent and received just like you wanted, nothing more to report here.” But if the delivery couldn’t be completed, you’ll be notified with an error code instead.

There are dozens of these so-called bounce codes, each signifying a different type of issue. Not only do bounce codes tell you that something went wrong, but they also provide some details around what actually happened — which may help you immediately correct the issue.

Okay, so what does a bounce message actually look like?

All codes are three digits long and start with either a four or five, as follows:

  • 4.X.X bounce codes signify a soft bounce

  • 5.X.X bounce codes are used for hard bounces

For instance, in this notification for a code 5.0.1email bounce, we can see that the recipient’s address either couldn’t be found or wasn’t able to receive mail:

Address not found image

And because it’s a hard (AKA permanent) bounce, we know the mailbox provider isn’t going to try to deliver the email message again.

This bounce code is pretty clear cut: most likely, there’s either a typo in the email address, or it just doesn’t exist. So your next step would be obvious — check the address for obvious errors, and if you can’t find any, make sure you don’t try sending to the same address again.

But not all bounce codes are quite so easy to understand and respond to.

Let’s take a look at the most common SMTP error code types and explain what they mean:

Bounce codeBounce typeWhat it means
4.2.1Soft bounceEmail server encountered a temporary error. If you try to send the same message again with no change at your end, it may go through just fine.
4.2.2 4.5.0 4.5.1 4.5.2Soft bounceMail server encountered a temporary failure. Again, repeating the action may result in successful delivery.
5.0.1Hard bounceThe transaction permanently failed and won’t be reattempted. This error message is most often caused by trying to send to an incorrect or invalid email address.
5.4.0Hard bounceThe transaction permanently failed and won’t be reattempted, typically because the recipient’s account has been deactivated or suspended.
5.5.0Hard bounceThe transaction permanently failed and won’t be reattempted. This is used as a general error message and is commonly accompanied by a status code providing additional details.
5.5.1Hard bounceThe transaction permanently failed and won’t be reattempted. Different email providers use this code in different ways (e.g. ImprovMX sometimes uses it when the sending domain’s reputation is too low).
5.5.2Hard bounceThe transaction permanently failed and won’t be reattempted, typically because the recipient has exceeded their storage allocation.
5.5.3Hard bounceThe transaction permanently failed and won’t be reattempted. Reasons vary by service provider (e.g. Yahoo uses it when the sender has been blocklisted by Spamhaus).
5.5.4Hard bounceThe transaction permanently failed and won’t be reattempted. Again, this error code varies by service provider, but often relates to content being identified as spam.

Why Does Email Bounce Matter?

By this point, it should be clear that email bounce can be caused by a whole lot of factors.

But does any of it really matter? Is it such a big deal if you send the occasional email to an inactive address?

Unfortunately, the answer’s “yes”.

A bounce notification isn’t just a message telling you that one of your emails couldn’t be delivered — it’s a warning that there could be something fundamentally wrong with your email campaigns, from the way you build prospect lists to your reputation as a sender. If you don’t look into and address the issue, it could come back to bite you where it hurts: your sender reputation and email deliverability.

Contrary to popular opinion, mail servers and inbox providers don’t hate bulk senders.

In fact, they’re kinda like baseball umpires. A single swing-and-a-miss needn’t have lasting consequences. But if you continue to ignore those bounce notifications and don’t take action to correct the underlying issues, you’re outta here!

Which makes sense, because their job isn’t to deliver your cold emails — it’s to keep their users happy. If you send a bunch of emails to non-existent Gmail addresses, it’s a pretty clear indication to Google that there’s something shady going on with your list-building practices. Keep it up and, sooner or later, Google will take action. In the worst-case scenario, they could even decide to block your emails to totally legit Gmail addresses.

Clearly, you don’t want that to happen, so it’s in your best interests to maintain the lowest possible bounce rate.

But what does “acceptable” look like in the world of cold outreach?

What's an Acceptable Email Bounce Rate?

While any email bounce is a cause for concern, you should aim for a maximum bounce rate of 4% — and ideally significantly lower.

Worryingly, it seems a lot of cold emailers are struggling with this. In fact, when we analyzed data from 65 million journeys and emails sent via QuickMail, we discovered that theaverage cold email bounce rate is actually 7.5%.

In other words, for every 100 cold emails sent, 7.5 end up returning a soft or hard bounce.

That’s too high. Clearly, a lot of senders aren’t just jeopardizing their deliverability — they’re also wasting a ton of time on low-quality prospecting and/or spammy messaging.

3 Best Practices To Prevent Email Bounce

It’s almost impossible to totally eliminate email bounces as a cold outreach professional, but there are definitely steps you can take to minimize them:

Use an Email Verification Tool

As you can see, a lot of bounces are caused by issues with the sender’s email address.

The good news is that you can cut down on these errors by using an email verification tool like Dropcontact or NeverBounce.

email verification tool image

Pro tip: Both Dropcontact and NeverBounce have direct integrations with QuickMail, allowing you to validate email addresses before launching a campaign. Nice!

Learn more: Adding Email Verification - QuickMail Knowledge Base 

Both tools assign labels to each email address you attempt to verify. These labels speak to the risk attached in sending to a given address. Common labels include:

  • Good: The email is valid and safe to send to.

  • Risky: The verification tool can’t verify whether the email is valid.

  • Unknown: The email address exists, but it might not be a personal inbox.

  • Invalid: The email address definitely doesn’t exist.

Of course, only sending to “good” emails will rule out a lot of “risky” and “unknown” addresses that are potentially legit, thereby limiting the reach of your campaigns. But remember: even a handful of bounces could have big implications for your sender reputation. We just don’t think it’s worth gambling with the deliverability of your future emails.

Pro tip: With QuickMail, you can automatically reject risky, invalid, or unknown emails from starting on a campaign, protecting your bounce rate while saving you the time and effort of weeding them out manually.

Look Out for Bounce Rate Spikes

We’ve already noted that, as a general rule, you should be aiming for a maximum bounce rate of 4% (and ideally lower).

But the total number isn’t the only thing to consider. You also need to factor in the performance of your typical campaigns.

For instance, if you generally see a bounce rate of ~1.5% and it suddenly leaps up to 3%, that’s a strong indicator that something’s going on. So you’d definitely want to look into it right away, rather than waiting to see if your bounce rate hits 4%+.

Analyze the Types of Bounces You’re Generating

There are lots of reasons why emails bounce — and some are more serious than others. As such, it’s important to look at not just the total volume of bounce notifications you receive, but also the reasons behind those bounces.

If you’re a bulk sender, you likely won’t have time to investigate every single email bounce; there just aren’t enough hours in the day. But you should pay particular attention to bounces that fall into one (or both) of these categories:

  1. Hard bounces, which are generally more serious than soft bounces.

  2. Any bounces related to spam or reputation, whether hard or soft, because they likely indicate broader issues that could hamper your email deliverability.

Also, if you’ve received multiple bounces from a single email address, it’s definitely time to remove that recipient from your email list. Otherwise you risk getting more of your emails blocked.

Check Your Emails Using SpamTester

One of the simplest and most effective ways to reduce the chances of your emails bouncing is to run them through a tool called SpamTester, which awards a score out of 10 based on factors like the contents of your message and the trustworthiness of your domain.

SpamTester example

The higher your score, the less likely you are to land in the spam folder.

And best of all, SpamTester is totally free to use!

Maximize Email Deliverability With QuickMail

The whole point of cutting down on bounced emails is to boost your email deliverability — meaning more of your emails reach the main inbox rather than lingering in spam folders (or, even worse, not being delivered at all).

That’s where QuickMail comes in.

We’re obsessed with email deliverability (in a good way). That’s why every QuickMail plan includes free auto-warmer access…

…and it’s also why we offer inbox rotation, meaning you can spread email sends across multiple inboxes. That way, if one inbox sees a drop in deliverability, you can switch to a different one.

But don’t take our word for it.

See what QuickMail does for your deliverability by signing up for your free trial today!