SMTP
451 Local Error
The requested action was aborted due to a local error in processing. The server encountered an internal issue that prevented it from completing the command.
When You See This Code
When the receiving server hits an internal error such as a database failure, DNS lookup timeout, or anti-spam filter processing error.
How to Fix
Retry the message later. If you control the server, check its logs for the specific error — common causes include DNS resolution failures, disk I/O errors, or database connectivity issues.