Are You Getting Email Returns? Here's Why...

Ever had an email “bounce back” and wondered why? There is a specific protocol for informing you when messages do not reach the intended recipient (for many reasons).

Being familiar with how this process works can help you determine the issue and why the email was returned to you.

What causes these messages?

Here are a few of the reasons:

The user gave an incorrect email address – usually a typo.
The user account no longer exists (or is experiencing technical difficulties).
The email box is at capacity (over quota).
A spammer intentionally gave an incorrect return address.
Automated messages (autoresponders) are also susceptible to the above reasons.

An undeliverable or bounce-back message will always explain why it could not be delivered in the return message. The return message you receive will also include an undeliverable error code. A typical return looks something like this:

The original message was received at Thur, 28, May 2026 18:45:05 -0500
(EST) from domain.isp.net [207.69.XXX.XXX]

—– The following addresses had permanent fatal errors —–

—– Transcript of session follows —–
… while talking to mx.domain.or.xx.:
>>> RCPT To: [fake, wrong or erroneous email will be here]
<<< 550 … User unknown
550 … User unknown
</END>

Messages Will Vary Due to Systems

These messages vary depending on the systems involved in email delivery. For example, if you have automated responses engaged. When a spammer sends you an email, your autoresponder will likely bounce back due to the spammer’s bogus address.

At the top of each undeliverable email is a code indicating why the network was unable to deliver it. Here is a list of some of the most common error codes you may encounter when you find bouncebacks in our inbox.

421 Service not available, closing transmission channel
450 Requested mail action not taken: mailbox unavailable
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage (mailbox full)
550 Requested action not taken: mailbox unavailable (mailbox not found, no access)
552 Requested mail action aborted: exceeded storage allocation (mailbox full)
553 Requested action not taken: mailbox name not allowed (mailbox syntax incorrect)
554 Transaction failed (timed out, resources, network issues)

Many undeliverable emails are due to a simple typo in the email address. Second to that comes spam. And if your email looks spammy, it may trigger spam filters, causing the server on the other end to reject it.

In most cases, when you take the time to look at the top of the returned email, you will find exactly why the email bounced. Then you know what action, if any, you need to take.