Permanent failure (hard bounce)
Email error 550 5.2.1: Mailbox disabled or not accepting mail
The mailbox exists but is turned off or set not to receive mail. It is a permanent rejection for that address.
Common causes
- The account was suspended or closed.
- The person left and the mailbox was disabled.
- The address is a list or alias that no longer accepts messages.
- The administrator blocked the mailbox on purpose.
How to fix it
- Stop sending to the address.
- Remove it from your active lists.
- Find a replacement contact if the person still matters to you.
- Verify the list so disabled mailboxes are removed before sending.
How to prevent it
Remove the address from your list and verify the rest before the next campaign.
Clean a whole list · MX lookup · SPF, DKIM and DMARC checker
Frequently asked questions
Is it different from user unknown?
Yes: the mailbox exists but is disabled. The result for you is the same, remove it.
Can it come back?
Rarely. Do not keep sending in the hope that it does.
Other error codes
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