A permanent refusal. Either the mailbox does not exist, or the server rejects you by policy: spam, blacklist, SPF/DMARC failure, or relaying denied. The enhanced code and text after 550 tell which.
550
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 5321
What to do
If it says “user unknown” or 5.1.1: the address is wrong; check spelling.
If it mentions spam, block, blacklist or 5.7.1: check your IP on blacklists and your SPF, DKIM, DMARC.
If it says “relay not permitted”: your server is not allowed to send for that domain, authenticate first.
How it appears in a bounce
550 Mailbox unavailable / rejected
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.