SMTP 502 Command Not Implemented vs 551 User Not Local
Both SMTP 502 (Command Not Implemented) and 551 (User Not Local) belong to the 5xx Permanent Negative category. 502 indicates that the command is recognized by the SMTP specification but is not implemented on this server. The server understands the command but has chosen not to support it. 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 command is recognized by the SMTP specification but is not implemented on this server. The server understands the command but has chosen not to support it.
When You See It
When using optional SMTP commands like VRFY, EXPN, or TURN that the server administrator has disabled, often for security or anti-spam reasons.
How to Fix
Use an alternative approach — for example, if VRFY is disabled, send a test email instead of verifying the address. Check the server's EHLO response for supported extensions.
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 502: The command is recognized by the SMTP specification but is not implemented on this server. The server understands the command but has chosen not to support it.
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 502 when when using optional SMTP commands like VRFY, EXPN, or TURN that the server administrator has disabled, often for security or anti-spam reasons.
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 502 (Command Not Implemented): Use an alternative approach — for example, if VRFY is disabled, send a test email instead of verifying the address. Check the server's EHLO response for supported extensions. 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.