SMTP 550 5.1.0: sender address rejected (bad from or return-path)
A 550 5.1.0 reply means the receiving server rejected the sender address on your envelope, usually the MAIL FROM or return-path. RFC 3463 defines X.1.0 as "Other address status", a catch-all for an address problem the server could not describe more precisely. The fault sits on the sending side of the exchange, not with the recipient.
The enhanced status code 5.1.0 is defined by RFC 3463 as "Other address status": something about an address in the transaction triggered the bounce, and the server had no more specific code to return. In practice the address it objected to is your sender or return-path, not the recipient.
Common causes
The MAIL FROM address uses a domain that does not exist or has no valid DNS.
The return-path is malformed, for example a missing domain or a stray character.
A policy on the receiving side rejects null or mismatched sender addresses.
How to fix it
Check the envelope sender your server sends in MAIL FROM and confirm the domain resolves in DNS. If you send on behalf of another domain, make sure the return-path is an address you actually control. Because the leading digit is 5, this is a permanent failure and the same envelope keeps bouncing until the sender address is corrected.
Common questions
Is 550 5.1.0 about the sender or the recipient?→
The sender. RFC 3463 X.1.0 is a general address fault, and receivers use 5.1.0 for a bad or rejected envelope sender, not a missing mailbox.
Is 550 5.1.0 permanent?→
Yes. The leading 5 marks a permanent failure, so the same envelope sender keeps bouncing until you fix it.
How is 5.1.0 different from 5.1.1?→
5.1.1 points at a missing destination mailbox. 5.1.0 is a broader address fault that receivers usually apply to the sender or return-path.
What should I check first?→
Confirm the MAIL FROM domain resolves in DNS and the return-path is an address you control.