First off I'll have to say that sending 20M of attachments is an extremely bad idea. You are much better off sending multiple emails, and in all honestly for that volume of data email is a very poor transport. You are far better served by emailing links to the documents that are left on your server. I know many corporate email systems will reject emails larger than 10M. It's a very common practice. With that in mind you may need to deal with the possibility what you're trying to do will not be possible at all.

What I mean by on-server path is this. If you're sending to person@destination.com Outlook Express will connect to smtp.something.com which is your company or ISP  mail server. On your PC there may be a virus scanner that scans the emails coming inbound, outbound, or both (Symantec proxy). That scanner may or may not scan for all email clients, and it may not scan outbound for known clients (Outlook Express) and scan for unknown clients (local web site, etc) that could be infected with virus's. So the path is that case is client->proxy->smtp.something.com->smtp.destination.com. During your send you will only receive an error from the proxy, if smtp.something.com or smtp.destination.com rejects the email you won't get an error but you may get a email bounced back to the sending or reply-to address.

If you're running the asp.net on a server the path would be web site->smtp.something.com->smtp.destination.com or possibly web site->localhost->smtp.something.com->smtp.destination.com. Again. in this case you only get an error returned to your application if an error is generated by the server you connect to (either localhost or smtp.something.com). If any other server rejects your email you'll get a bounce, or potentially nothing.