Error "<#5.5.2 smtp;554 5.5.2 Invalid data in message> #SMTP#" when you send attachment in your mail
Some users were receiving error <#5.5.2 smtp;554 5.5.2 Invalid data in message> #SMTP# when they sent message with attachment.
The problem is when attached document name is longer than 50 characters. Some Firewalls have header size restriction and that limit blocks this messages to be sent.
To correct this, change header limit on affected Firewall or shorten the name of the document you are sending (which is probably not good permanent solution).
Update #1: Default header size limit in Exchange 2007 is 64K.
Update #2: It might be the SMTP version problem. Mail is rejected with CONTENT-DISPOSITION error (overly long message header field for CONTENT-DISPOSITION) but not for all SMTP servers (same mail sent through different SMTP servers). If you send mail with attachment that has name longer than 50 characters from Exchange 2007 directly through DNS you may encounter this error, but if you send same mail from Exchange 2007 through smart host (other than Exchange) mail is delivered with no errors (possible workaround).
Update #3: There is a good article on Symantec's web site about parameters in Config.cf file. Check SMTP block/unblock possibilities.
Update #4: Anonymous (*send me your name*) left a comment below regarding reasons for this error message:
"We found this was simply a matter that the message was created in Outlook "rich text" format (which embeds attachments into the message body) instead of plain text or HTML which creates attachments normally"