550: a permanent rejection
By Jose Pollman · Published · Updated
550 is a permanent rejection, and without an enhanced code it says nothing more. The text after the number carries the reason, which is why two 550s can need opposite responses. If it mentions the user or mailbox, the address is wrong or gone, so remove it. If it mentions policy, blocking or spam, treat it as reputation.
The error
550 Requested action not taken: mailbox unavailable
550 5.0.0 RejectedWhat it means
A permanent rejection. Without an enhanced code, 550 covers everything from a non-existent address to a policy block, so the text of the bounce is what identifies the cause.
Something about the message or the address you sent to needs changing.
Why it happens
RFC 5321 gave 550 a single meaning — the requested action was not taken — and everything since has been bolted on. Receivers that never adopted the enhanced status codes of RFC 3463 still express every permanent refusal through it, so the number narrows the cause to "permanent" and nothing more. That is why the prose after the code carries all the information, and why two bounces with the same number can need completely opposite responses.
How to fix it
- Read the text after the code — it is the only thing that says why.
- If it mentions the user or mailbox, the address is wrong or gone; remove it.
- If it mentions policy, blocking or spam, treat it as a reputation problem.
Related guides
- 5.1.2: the recipient domain does not exist — The domain in the recipient address cannot be found in DNS, or has no mail servers. The problem is the part after the @.
- 5.4.6: a mail routing loop — The message was passed between servers in a circle until the loop was detected. Two systems each believe the other is responsible for the domain.
- 550 5.1.1: the mailbox does not exist — The address does not exist on the receiving system. This is the single most common bounce there is.