We have noticed a bug in the case where an email subject ends in .com (eg. This is a test@microsoft.com), and a user attaches that email (as an attachment on a new message). If there is a Transport Rule set up to block emails by file extension and it includes the .com extension - it will block the email based on the attachment even though .com is not the file extension (it's actually .msg).

To recreate the problem in Office 365:

Using Outlook or OWA:
1. Send yourself an email with the subject line "this is a test@test.com"
2. Open a new email message and drag/drop the first email as an attachment onto the new email message.
3. Try to send the message, if you have a Transport Rule enabled which blocks the .com file extension, it will block this email attachment even though it's a .msg file, not a .com

This is the case for any file type specified, if you type the subject of the email to be "This is a test@test.bat", it will block that as well if the .bat file extension is in the Transport rule, even though it's a .msg file.

The only way to get it to work is to use OWA, and save the email to your PC first, then use the Add Attachment option on the new message window - then it will show as a .msg file properly and the email will go through. This same procedure does not work in Outlook, nor does dragging/dropping the email from Outlook or OWA.

Hope this makes sense - it's a bug in my opinion.