Outlook Express has been obsolete for so long I can't even remember the last time I actually saw anyone using it... It's a security nightmare, and has been removed from Windows ages ago
AFAIK, Outlook Express doesn't have an API to manipulate it programmatically (but Outlook does).You should probably send the mail directly from your C# code, without involving Outlook Express. Check out the System.Net.Mail namespace in MSDN, it contains everything you might need (including code samples) or use Outlook via VSTO.
I was wrong, check here
Reading and Writing Messages in Outlook Express
and also this