550 5.1.0 "Address Rejected": What It Means and What to Do With the Address

550 5.1.0 means the receiving server rejected the address and did not say why. The text is usually "Address rejected." and in Exchange Online it can read "Sender denied." Treat it as a permanent address problem: check the recipient, and if the address is right and still fails, drop it from the list.

The 550 5.1.0 code page has the quick version.

Reading the code

Part	Value	Meaning
Reply code	550	Requested action not taken, mailbox unavailable
Class	5	Permanent failure
Subject	1	Addressing status
Detail	0	Other or undefined address status

RFC 3463 defines X.1.0 as "other address status". It is the catch-all for address problems with no sharper code. If the server had a reason, it would have used a code like 5.1.1 user unknown or 5.1.10 recipient not found.

What causes it

• The recipient does not resolve on the receiving server, and the server did not say why.
• In Exchange Online, a reply was sent from an old or offline copy of a message whose recipient no longer resolves.
• The sender address is being refused by the receiving server.

What to do

1. Check the address for typos and resend to the correct one.
2. In Outlook, type the address fresh instead of picking an old entry from auto-complete.
3. If it is right and keeps failing, treat it as dead and remove it. Removing bounced emails from a list shows how.
4. Look at the pattern. If many 5.1.0 rows come from one source or domain, find which list source caused the bounces.

Hard bounce or not

5.x.x codes are permanent, so this is a hard bounce. Hard bounce vs soft bounce covers why senders suppress them.

How verification prevents it

This is one of the address-related codes a list check can catch before you send. A mailbox check asks the receiving server about the address, so an address that cannot resolve comes back invalid with a reason such as "Mailbox not found" instead of reaching your sequence as a 5.1.0 bounce. Servers that accept every address come back as "Accepts all addresses", so those rows need judgement rather than a yes or no.

Upload your list on the verify page: the first 100 email checks are free with a Google sign-in, no card. Nothing is sent to anyone on the list. For larger lists, see bulk email verification, list cleaning and pricing.