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:

owner-nolist-MORNING_BRIEF_TEXT-080219O-p70w0se3*PSHELTERS**ERAC*-COM@RETURNS.DOWJONES.COM

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.