Hi, I have an installation on Exchange Server 2003, running Mail Security 4.3.10025.0 The customer complained that genuine email messages were getting bounced. I have traced this to Transport Agent (turning OFF solves the issue). The error message is : host said: 554 5.6.0 Invalid content (in reply to end of DATA command) The email message I have been experimenting with appears a fairly normal message, but has two MS Word attachments ( .doc ). I assume it's not safe to leave Transport Agent OFF, so have checked it on again. Does anyone have any idea whether I can tweak the Mail Security, or any other solution ? Thanks. Note that I received the same message ok from the same sender, using Pop email, not through the Exchange Server with Mail Security. I then saved the two MS Word documents, and sent a new message to the recipient with the Word documents attached - that message successfully was delivered (with Transport Agent enabled). Which suggests that the problem lies in the original message, not necessarily in the attachments.