SMTP 455 Server Unable to Accommodate vs 502 Command Not Implemented
SMTP 455 (Server Unable to Accommodate) is a 4xx Transient Negative response, while 502 (Command Not Implemented) is a 5xx Permanent Negative response. 455 indicates that the server is currently unable to accommodate the requested parameters. This is a general temporary failure indicating the server cannot process the specific request at this time. In contrast, 502 means 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.
Description
The server is currently unable to accommodate the requested parameters. This is a general temporary failure indicating the server cannot process the specific request at this time.
When You See It
When the server temporarily cannot handle a specific aspect of your request, such as a particular extension or parameter that requires resources not currently available.
How to Fix
Retry the command later or try with different parameters. If the error persists, simplify the request or contact the server administrator to check resource availability.
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.
Key Differences
455 is a 4xx Transient Negative response, while 502 is a 5xx Permanent Negative response.
SMTP 455: The server is currently unable to accommodate the requested parameters. This is a general temporary failure indicating the server cannot process the specific request at this time.
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.
You encounter 455 when when the server temporarily cannot handle a specific aspect of your request, such as a particular extension or parameter that requires resources not currently available.
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.
When to Use Which
For 455 (Server Unable to Accommodate): Retry the command later or try with different parameters. If the error persists, simplify the request or contact the server administrator to check resource availability. 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.