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Message Blocking & Attachments:

Why are some mail attachments 'blocked' by the mail system?


The GroupShield Anti-Virus software on our Exchange server site has always had the ability to block certain types of message attachments. Unfortunately this feature didn't initially work very well. Because this feature now works within the version of GroupShield installed in August 2002, this issue was re-examined.

During August 2002, our site received over 500 contaminated attachments (and August is one of our slow months). Upon examination of the quarantine log and review of the most active viruses, the following attachment types (file extensions) are now blocked by the Montana Tech mail system:
EXE COM BAT SCR PIF RAR VBS

This list may change based on future virus vulnerabilities.

If you need to receive or send a mail attachment using one of these blocked file extension types, we recommend you either:

A) Compress the source files using the WINZIP file utility:

The ZIP file extension (created by the WINZIP utility) is NOT currently blocked by default. Also, executable files tend to be large by nature, thus it is always good practice to compress them before sending them via email anyway.

B) Rename the source file:

From a blocked file type (listed above) to a benign type (such as .TXT) before attaching/sending. The message receiver must then rename the attachment back to the original extension type after reception.

If the mail server AV software detects an infected attachment it will AUTOMATICALLY replace that attachment with a notification attachment called either "Replaced_Infected_File.TXT" or "Blocked_Infected_File.TXT". Our Anti-Spam appliance is also set to block messages with attachments of the types listed above.