Bounced emails: why an email bounces, what each error means and how to fix it

Did you get a “failure notice” or “couldn’t be delivered” message after sending a campaign?

That’s a bounced email: a message that didn’t reach its destination.

The recipient’s server sends it back to you with an error code.

Some bounces fix themselves, like a full mailbox. Others are permanent, like an address that no longer exists.

Telling them apart is what lets you clean your list without losing valid contacts and without damaging your sender reputation.

In this article you’ll find what each bounced email notice means and the most common error codes.

You’ll also see Mailrelay’s table of bounce reasons and how to handle them step by step.

What a bounced email is

A bounce happens when the recipient’s mail server rejects your message. Instead of delivering it, it sends back a notice with the reason.

That notice goes to the sending tool, which classifies it.

If you send from your own email account, you see bounced emails as error messages in your inbox.

There are two main types: the hard bounce, which is permanent, and the soft bounce, which is temporary.

You’ll find each type explained in depth in the hard bounce and soft bounce entries.

What the most common bounce notices mean

Each email provider words the notice in its own way. These are the ones you see most often and what they usually mean:

What matters isn’t the subject of the notice, but the error code inside it.

Most common SMTP bounce codes

Codes that start with 5 are permanent errors. Those that start with 4 are temporary.

These codes follow the mail system status code standard (RFC 3463).

Gmail and Microsoft also publish their own on their help pages.

How to read a bounce notice, with an example

A bounce notice looks complicated, but you only need to look for three things: the address, the code and the explanation.

This is a typical example of a hard bounce:

Subject: Delivery Status Notification (Failure)

Your message couldn’t be delivered to [recipient address] because the address doesn’t exist.

Remote server returned: 550 5.1.1 The email account that you tried to reach does not exist.

The address is the one shown in the notice. The code, 550 5.1.1, starts with 5, so it’s a permanent error.

And the explanation confirms it: the account doesn’t exist. There’s no point sending to it again until the person gives you a correct address.

If the code started with 4, like a 452 4.2.2 for a full mailbox, waiting would be enough.

What to do if an email you sent from your own account bounces

Not all bounced emails come from campaigns.

Sometimes it’s a regular email you send from Gmail or Outlook to a customer or a supplier.

In that case, first check that the address is spelled correctly.

It’s the most common cause, especially with domains like “gmial.com” or “outlok.com”.

If the address is correct, look at the code in the notice.

A full mailbox sorts itself out over time. A policy error usually means the destination server mistook you for spam.

In that last case, call the person and ask them to check their spam folder or add you to their contacts.

And if the notice says your own message is too large, remove heavy attachments and send a download link instead.

Bounce reasons in Mailrelay and how to fix them

In the Mailrelay dashboard, every bounced email appears with a code that sums up the reason.

This is the full table, with what you can do in each case:

When the address works again, write to Mailrelay support to reactivate the subscriber.

How Mailrelay flags bounces

When a send gets a hard bounce, the contact is flagged as bounced and Mailrelay doesn’t send it anything else.

It isn’t unsubscribed. Unsubscribing is something only the subscriber can do, and a bounce isn’t that.

With soft bounces, Mailrelay keeps trying to deliver. There’s no fixed number of soft bounces after which sending to a contact stops.

Some bounces arrive hours after the send. These are asynchronous bounces, and to catch them you should turn them on when you set up your custom domain.

Handling bounced emails in 6 steps

If you see more bounced emails than usual after a campaign, follow this order:

1. Separate hard bounces from soft bounces in the send report.
2. Check the hard bounces for obvious typos, like “gmial.com” or “hotmial.com”.
3. If the address matters, contact the person another way, by phone or in person.
4. Keep an eye on soft bounces: if an address bounces in several campaigns in a row, treat it as suspicious.
5. If the bounces are spam- or policy-related, check your authentication, your content and your reputation.
6. Look at where the bouncing contacts come from to fix the source of the problem.

The last step is the one most often forgotten.

If the bounced emails always come from the same form or the same import, that’s where the problem is.

How to prevent bounces before you send

The best bounce management is the kind you don’t need. Almost all bounced emails can be avoided with a well-maintained list.

Use double opt-in so every subscriber confirms their address. That way, misspelled or made-up addresses don’t get in.

Before importing an old list, verify the emails and remove the ones that don’t exist.

And never buy databases: they’re full of expired addresses and spamtraps.

Set up SPF, DKIM and DMARC on your domain. Without them, Gmail and other providers may block your sends even if the addresses are correct.

Finally, keep up your list hygiene with regular reviews. Contacts who haven’t opened your emails for months end up bouncing too.

Why bounces affect your deliverability

Email providers keep track of how many bounced emails you rack up. A sender who sends to lots of non-existent addresses looks like someone who doesn’t take care of their list.

That’s why a high bounce rate damages your sender reputation.

And with a worse reputation, more emails end up in spam, even those going to valid addresses.

As a rough guide, if your hard bounces go above 2% in a campaign, check where that list came from before your next send.

You’ll find more detail in the guide to deliverability in email marketing and in why you should remove failed accounts from your list.

How Mailrelay helps

Mailrelay classifies every bounced email with its reason and stops sending to addresses with a hard bounce, without you having to do anything.

In each send’s reports you see bounced emails separately from the rest, and every signup goes through double opt-in.

If you have questions about a specific bounce, human support helps you on every plan.

The free account lets you send up to 80,000 emails a month to 20,000 contacts. You can see all the plans on the pricing page.

Frequently asked questions about bounced emails

These are the most common questions when an email bounces.

What does it mean when an email has bounced?

It means the recipient’s server didn’t accept it and sent it back to you with an error notice.

The reason can be temporary, like a full mailbox, or permanent, like an address that doesn’t exist.

What is a “failure notice”?

It’s the subject line some servers, like Yahoo, use to tell you an email couldn’t be delivered.

The notice contains the error code that explains the reason.

What does bad-mailbox mean?

It means the mailbox is wrong: the address doesn’t exist or is misspelled. It’s a hard bounce.

Does a bounce unsubscribe the subscriber?

No. In Mailrelay the contact is flagged as bounced and nothing more is sent to it.

Unsubscribing is something only the subscriber does.

Can a bounced contact be recovered?

Yes, if the address starts working again. Check that it’s correct and write to Mailrelay support to reactivate it.

Jose Argudo

CMO at Mailrelay. Writes about email marketing, copywriting and conversion for SMEs and entrepreneurs.

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