SMTP 221 Closing Connection vs 455 Server Unable to Accommodate
SMTP 221 (Closing Connection) is a 2xx Positive Completion response, while 455 (Server Unable to Accommodate) is a 4xx Transient Negative response. 221 indicates that the server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session. In contrast, 455 means 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.
Description
The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
When You See It
After sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
How to Fix
No fix needed — this is the expected response when ending an SMTP session. If you see this unexpectedly, the server may be shutting down or timing out idle connections.
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.
Key Differences
221 is a 2xx Positive Completion response, while 455 is a 4xx Transient Negative response.
SMTP 221: The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
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.
You encounter 221 when after sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
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.
When to Use Which
For 221 (Closing Connection): No fix needed — this is the expected response when ending an SMTP session. If you see this unexpectedly, the server may be shutting down or timing out idle connections. 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.