SMTR-300 — The proxy has received line longer than protocol defined maximum
The line received from a network peer is longer than limits defined in particular RFC (RFC2821). The first word in the message explains whether the line was command line or mail data line.
The mail line length accepted by the proxy is 16 kB (which is definitely enough comparing to the RFC limit 1000 B).
The command line limit is 510 B.
Violation of any of these rules should be reported to the mailsender.
Command/mail cannot be processed and will be rejected.