Returned mail to User who didnt send mail

[philb101]

A User on a Customers Intenral Network reported on Monday that she received 1
18 emails reporting
"Subject: Mail delivery failed: returning message to sender
This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

btckgp@cbn.net.id *** Note: this varies
mailbox is full: retry timeout exceeded"

The Users PC is powered off every evening & all weekend
The Internal Network has its own Mail Server running CMailServer, this is left running all weekend but no email is created on the Mail Server
All incoming Mail is scanned by a Barracuda Spam Filter before being passed onto the Mail Server & then onto the User
The emails being returned are from addresses never used by the customer

Is there a problem on the Internal Network
Are there any actions I can take to prevent this

Looking for some inspiration
Thanks for all replies

[veedubfreak]

Someone she knows has a virus and its using mail spoofing.

[Swe~T]

So virus ids under her PC?
Or such confirmation emails content viruses?