This was an old post I started to write a couple months ago. Not sure why I didn't publish it, but considering the project in question has come to an end, I can pretty much call this issue "closed as unresolved".
I thought I was close in finally being able to send an email with an attachment using the default email client in C#. The code that I found worked great when I gave it a quick test on my work machine with XP Thunderbird. It also worked on my development machine running Vista and Live Mail. But when I set up a vanilla XP virtual machine for testing, Outlook Express failed with a message "One or more parts of this message could not be displayed".
Well, at the very least, I could insert the code, and make it a configurable option so I could switch back to the old crap way of using a mailto: link if it doesn't work for the client.
Just my luck, the client recently replaced his old desktop PC with a Mac, and he had his own vanilla XP virtual machine for running Windows software. Sure enough, the mail code failed just as spectacularly in that version of Outlook Express as it did on mine.