[Mailman-Users] error reading SMTP packet; response to dot-stuffed message expected"

[Sony Z Thekkumthara]

Any one can please shed some lights on this error message...
"error reading SMTP packet; response to dot-stuffed message 
expected"

This happens for a few users who are using their ISP to post a 
message to our mailman list server. Some times the mails delays 12 
hours or more or will not reach at all.

Thanks so much,
Zach