SMTP 535 Authentication Failed vs 551 User Not Local
Both SMTP 535 (Authentication Failed) and 551 (User Not Local) belong to the 5xx Permanent Negative category. 535 indicates that the authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server. Meanwhile, 551 means that the recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
Description
The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
When You See It
After submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
How to Fix
Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.).
Description
The recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
When You See It
When sending to a recipient that has moved or whose mailbox is hosted elsewhere, and this server refuses to forward the message to the new location.
How to Fix
Check the server's response for a forwarding address and resend directly to that address. Update your contact records if the recipient has permanently moved to a new mail server.
Key Differences
SMTP 535: The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
SMTP 551: The recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
You encounter 535 when after submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
You encounter 551 when when sending to a recipient that has moved or whose mailbox is hosted elsewhere, and this server refuses to forward the message to the new location.
When to Use Which
For 535 (Authentication Failed): Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.). For 551 (User Not Local): Check the server's response for a forwarding address and resend directly to that address. Update your contact records if the recipient has permanently moved to a new mail server.