Email being sent through smtp that keeps being quarantined.

My Symantec antivirus for exchange keeps send me an email that says:

Location of the message: SMTP
Sender of the message: postmaster@xxxxxxxxxxxx Subject of the message:
None

The attachment "pd75001944778.msg" was Quarantined.

This was done due to the following Symantec Mail Security settings:
Policy: Standard
SubPolicy: Exception SubPolicy
Rule: Unscannable File Rule

I am getting this about 20 times in a day. What is the msg that is
being sent? Who would be sending it and how can I get rid of it?

Any help would be appreciated.

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