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Understanding Undeliverable and Returned Emails

Why did my email bounce?  Here's why...

Do you get undeliverable email returns or bounce-backs? Do you wonder what they would mean? You are not alone. Here is the information you need to decipher why specific emails are undeliverable and returned.

Reasons Why Emails are Undeliverable

The several common reasons you may receive undeliverable email returns:

  1. You had a typo in the email address making it incorrect and therefore undeliverable. Conducive to dialing an wrong phone number.
  2. The person you are emailing actually gave you an incorrect email address or you made a typo – that happens a lot. That’s why it is not such a bad idea to have your website contact forms have visitors enter their email address twice.
  3. The recipient’s inbox is filled to capacity due to large attachments, server settings or not logging in for a while. What many times causes this problem is the user has “leave mail on server” checked in their email program. This setting prevents downloaded emails from being cleared. All email is “left on the server”. Until that option is unchecked then all email is downloaded this will continue to happen. (Leave this option unchecked unless you can micro-manage it.)
  4. Your autoresponder message could not “auto respond” to a bogus email address that a spammer used to email you.
  5. Your email was “spammy” and set off spam or network security filters.
  6. Someone who has your email address on their system has a virus that is sending to non-existent email addresses and putting your address in the FROM: field. This causes undeliverable virus generated emails to be return to you.

Decifering Returns

There is a protocol to let you know when messages do not (for many different reasons) make it to the intended recipient. Therefore, you want to become familiar with how these messages display precisely what the problem is.

When an email bounces, the reason why is noted at the very top of the message. If you look closely at the returned message, you will see what the problem is. It will look similar to this:

The original message was received at Wed, 06 Oct 2021 10:38:48 -0700 (PDT)
from adsl-157-24-41.XXXXX.XXXXXX.net [12.345.67.89 ]

—– The following addresses had permanent fatal errors —–

(reason: 550 5.1.1 … User unknown)

—– Transcript of session follows —–
… while talking to businessemailetiquette.com.:
DATA
550 5.1.1 … User unknown

The above reflects no such email address on that system (businessemailetiquette.com) – 550 user unknown.

You will see the actual email message that the error refers to is displayed below the error message. There you can determine if, in fact, it was an email you sent or if it was your autoresponder. In addition, you can decide if it was a virus-generated email that you did not send.

Email Error Codes

Here is a listing of the most common error codes you will see in undeliverable emails and what they mean:

  • 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 ( Example: 554 5.7.1 Forbidden for policy reasons — Your email was identified as spammy.)

Now You Know

These messages will vary depending on the systems involved in the delivery of the email. However, it is worth becoming familiar with these basics (or bookmarking this page) so that you don’t panic and can take action to address what you can.

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