What does 550 mean?

You sent an email, and your ESP came back with a 550. Now what?

A 550 is the most common hard bounce code in SMTP. It means the receiving mail server permanently rejected your message. The mailbox isn't there, isn't accepting mail, or the server's policy blocked you outright.

The most common reason is simple: the address doesn't exist. The account was never real, was mistyped at signup, or the person left the company and their inbox was deleted. You'll usually see a message like "550 User not found", "550 No such user here", or "550 5.1.1 Recipient rejected".

Other causes include a disabled account (the user's still employed, but IT shut off their mailbox), a policy block (the receiving server flagged your IP, domain, or content), or a full policy rejection at the organizational level.

Here's the key thing about 550: it's permanent. Unlike a 4xx code that tells you to try again later, a 550 is the server saying "don't bother". Retrying will not fix it. Worse, repeatedly sending to addresses that return 550s is one of the fastest ways to damage your sender reputation.

What to do when you see 550s

• Suppress the address right away. Don't retry, don't recycle it into a future campaign.
• Look at the full bounce message in your ESP. The extra text after "550" often tells you whether it's a missing user (a list quality problem) or a policy block (potentially a reputation or content problem).
• If you're seeing 550s at a volume above 2% of sends, that's a signal your list needs cleaning before your next campaign.

If your bounce report is full of 550s and you're not sure where they came from, it may be worth running your list through a cleaning pass. We do exactly that at RME (hi ;)) and we can flag addresses likely to return hard bounces before you send to them.