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Understanding Undeliverable Emails

Understanding email returns and bounce-backs

You check your email, and you find an “undeliverable email” in your inbox. So what the heck is going on? Well, let’s figure that out, shall we?

There are a bunch of reasons why emails cannot reach their intended destination. Consequently, some are out of your control to control.

Common reasons you may receive undeliverable email returns:

  • You had a typo in the email address, making it incorrect and undeliverable. Conducive to dialing the wrong phone number.
  • The person you are emailing gave you an incorrect email address (typo).
  • The recipient’s inbox is at capacity due to large attachments or not clearing their inbox. (Email settings can be the cause of this.*)
  • A spammer used a phony email address when sending it to you, and your autoresponder message could not respond to the bogus email address.
  • Someone who has your email address on their system has a virus propagating itself to old or non-existent email addresses by inserting your address in the FROM: field.


The primary cause of an inbox hitting capacity is the “leave mail on server” checked in your email program. This setting does not allow that email account to be cleared because the email is “left on the server.”

Over time, emails accumulate. This will continue until that option is unchecked and all email is downloaded to clear out the email account. Leave this option unchecked unless you can micromanage it.

Never have all your devices set to “Leave mail on server.” If you do, you’ll never empty your inbox and could face emails bouncing due to capacity issues.

For example, you can set your cellphone to “Leave mail on server” and your PC or laptop set to download all your emails. Always have one device with this setting turned off.

With this approach, you know that you are just viewing it when you check email on your phone. You then download it off the server later to your PC when you check the email there.

It’s All in the Bounce-back Email

Undeliverable email messages are also known as bounce-backs. Did you know there is a protocol in place for this? This protocol lets you know when messages you send do not make it to the intended recipient.

When you become familiar with how these messages are structured, you can determine the exact problem. In the same vein, becoming familiar with the various reasons will help you understand what is going on.

There will always be an “undeliverable reason” at the top of the email with all returned emails. This error code will tell you why the message was not delivered.

Example Returned Email Header

If you look closely at the returned message, you will see the problem. The top of the message will look similar to this:

This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.
The original message was received at Wed, 2 Aug 2020 18:45:05 -0500 (EST) from providers.isp.net [207.XX.XX.XX] —– The following addresses had permanent fatal errors bl******@do****.com while talking to mx.servername.com.: >>> RCPT To: <<< 550 … User unknown

The above example reflects no such email address on that system. The User is unknown.

I am using the examples above. In addition, the actual provider’s domain, IP, and other side’s MX server (mail server) information reflecting the exact data for that email is displayed.

These messages vary depending on the systems and software involved in delivering the email. For example, you may have to click View > Message Source in some email programs to see all the background routing info and server error code.

Then some systems are simpler:

bl******@do****.com, ERROR_CODE :554, ERROR_CODE :5.7.1 Message blocked due to spam content in the message.

The above lets you know that 554 (transaction failed) and why. You were identified as spam or doing something spammy.

Of course, my readers don’t spam, right? In this case, you would then need to contact the recipient and get them to add you to their whitelist/approved senders/address book.

In all bounce-backs, below the error message, there will be minimally the header (Date, From, To, and Subject:) of the email that could not be delivered. This will help you determine if it was an email you sent. Or if it was your autoresponder or a virus-generated email that you did not send.

Listing of the Most Common Error Undeliverable Email Codes:

  • 251 User not local: will forward to 421 Service not available, closing transmission channel
  • 450 Requested mail action not taken: mailbox unavailable (E.g., mailbox busy)
  • 451 Requested action aborted: local error in processing
  • 452 Requested action not taken: insufficient system storage
  • 500 Syntax error, command unrecognized
  • 501 Syntax error in parameters or arguments
  • 502 Command not implemented
  • 503 Bad sequence of commands
  • 504 Command parameter not implemented
  • 550 Requested action not taken: mailbox unavailable (E.g., mailbox not found, no access)
  • 551 User not local
  • 552 Requested mail action aborted: exceeded storage allocation (mailbox filled)
  • 553 Requested action not taken: mailbox name not allowed (E.g., mailbox syntax incorrect)
  • 554 Transaction failed

Some of the codes above are technical and have more to do with the “system” than anything you can address. If you are unsure when you receive a bounce-back, provide your email provider a copy of that return message.

Bookmark This Page!

Now you can look at a returned email and understand what the issue is. You’ll also be able to determine if what happened is in your control to control.

For example, your typo versus mailbox filled scenario. Go ahead and bookmark this page right now to refer to it as needed.

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