The error code 501 appears when InterScan Messaging Security Suite (IMSS) server sends an email from a sender with a long address like the following example:
IMSS is designed to comply with RFC 2821 specifications, which limits the allowed number of characters on a sender's email address. The imposed limit is hard-coded in IMSS program.
RFC 2821 specifies the following rules for the parts of a sender email address:
- Local part - The maximum total length of a user name or other local part is 64 characters.
- Domain - The maximum total length of a domain name or number is 255 characters.
- Path - The maximum total length of a reverse-path or forward-path is 256 characters, including the punctuation and element separators.
- Command line - The maximum total length of a command line, including the command word, is 512 characters. The SMTP extensions may be used to increase this limit.
As a workaround, disable the IMSS compliance to RFC:
- Open the C:\Program Files\Trend Micro\IMSS\config\tsmtpd.ini file using Notepad or any text editor.
- Go to the SmtpServer section and add the following parameter CheckRcptLength=0.
A value of "0" disables the sender email address length checking while a value of "1" enables this function.
- Save and close the file.
- Restart the Trend Micro IMSS SMTP Service.