Any way to add a pre-encoded attachment in Outlook?

I have a program that uses Outlook for it's emailing, but have run into a slight problem. Most of the emails are generated automatically when certain transactions are made, which means that the user needs to wait until they are sent. Problem is, a couple of them have several large attachments. Thus, serious hang-time while Outlook encodes them. So, I was thinking that I could just pre-encode them and add the encoded text to the email. Anyone know of a way to do this via the Outlook object model? I really don't want to have to switch to generating raw SMTP if at all possible.

If its that large of an attachment it should be zipped first. Plus, are you extending the attachment file size limit or is it below the limit?