SMTP 550 is a hard failure that typically means a mailbox doesn't exist or delivery is not allowed. Learn causes and fixes.
SMTP 550 is a permanent rejection. It often means the recipient address doesn’t exist, the domain rejects your mail, or a policy block is enforced.
You should suppress addresses that return 550 repeatedly. If it’s a policy rejection, review authentication, reputation, and whether your sending aligns with recipient requirements.