Earlier quoted context omitted.
Unless you're dealing with most corporate mail systems.
I don't quite understand what you mean - the 20MB file would stand out on a mail server? I find that unlikely, unless they're running OpenBSD. Is the 20MB file attached to mail messages? That also seems unlikely, if only because that's a really stupid way to design a virus.
Also, what if the mail component were used to hide/archive the virus? Hide a virus attachment from someone to themself, then have some bootstrap code (Outlook/email client exploit, perhaps) that loads the email archived virus back onto the comp.