Manipulated MIME attachment
Test your virus scanner with this email. It has an EICAR test file attached to it (see EICAR test). But the attachment has been manipulated in a way, that fools many antivirus programms.
Any antivirus program should be able to detect the EICAR test file anyway because you can still save and execute the programm. If your emails are filtered at the email gateway, the virus scanner should detect the file attachment as a virus and not let the email through, at least not with the file attached.
Warning: If your provider or another mail gateway filters your emails for viruses, this email may never reach you.