A 550 5.1.1 reply means the receiving mail server has no mailbox for the address you sent to. RFC 3463 defines X.1.1 as "Bad destination mailbox address". The address is wrong, was deleted, or was mistyped.
The enhanced status code 5.1.1 is defined by RFC 3463 as "Bad destination mailbox address": the receiving server has no mailbox matching the recipient address.
Common causes
A typo in the local part or domain of the address.
The mailbox was deleted, renamed, or its alias was removed.
You are sending to an address that never existed.
How to fix it
Confirm the exact address with the recipient, check for typos, and remove the address from any list that keeps bouncing. Because the leading digit is 5, this is a permanent failure: retrying the same address will fail again.
Common questions
Is 550 5.1.1 permanent?→
Yes. The leading 5 marks a permanent failure; retrying the same address will fail again.
Does 550 5.1.1 mean I am blocked?→
No. It is an address problem, not a reputation or policy block. The mailbox does not exist.
Why did a message that worked before now get 550 5.1.1?→
The mailbox was likely deleted or renamed, or an alias was removed on the receiving side.
How do I confirm the address exists?→
Ask the recipient to confirm the exact address, and check for typos in the local part and domain.