SMTP 455 Server Unable to Accommodate vs 550 Mailbox Not Found
SMTP 455 (Server Unable to Accommodate) is a 4xx Transient Negative response, while 550 (Mailbox Not Found) 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, 550 means that the requested action was not taken because the mailbox does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
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 requested action was not taken because the mailbox does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
When You See It
When sending to an email address that does not exist on the destination server, or when the server's policy rejects your message (e.g., anti-spam, sender verification failure).
How to Fix
Verify the recipient's email address for typos. If the address is correct, the mailbox may have been deleted or your domain may be blocked — check your sender reputation and SPF/DKIM/DMARC records.
Key Differences
455 is a 4xx Transient Negative response, while 550 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 550: The requested action was not taken because the mailbox does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
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 550 when when sending to an email address that does not exist on the destination server, or when the server's policy rejects your message (e.g., anti-spam, sender verification failure).
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 550 (Mailbox Not Found): Verify the recipient's email address for typos. If the address is correct, the mailbox may have been deleted or your domain may be blocked — check your sender reputation and SPF/DKIM/DMARC records.