The 550 SMTP error is the most common hard bounce code and can mean many things. Learn how to read the diagnostic text to find the specific cause.
🔴
Permanent Failure (Hard Bounce)
SMTP Code 550
Category Generic Delivery Failure
Frequency Common
💬 What This Error Means
The receiving server permanently rejected your message. 550 is the most common hard bounce code and is almost always accompanied by a more specific enhanced status code (like 5.1.1 or 5.7.1) or a plain-English reason in the diagnostic text.
Common Causes
Mailbox does not exist (most common — see 5.1.1)
Policy rejection or blocklist (see 5.7.1)
DMARC, SPF, or DKIM failure (see 5.7.23, 5.7.26)
Domain has no mail servers
How to Fix This
Read the full diagnostic text in the bounce message for the specific reason
Look for an enhanced status code (X.Y.Z) and look it up for detailed guidance
Use our Domain Checker if you suspect an authentication issue
Use our Blacklist Checker if you suspect an IP or domain reputation issue
📋 Real-World Example Messages
These are real bounce message formats you might receive. Paste yours into the
Bounce Decoder for instant analysis.
550 Requested action not taken: mailbox unavailable
550 No such user here