Rather than use the Mail App, head to the web mail for Compuserve and check your sent folder there.
If the machine has some trojan app installed I usually find the sent mail at the web mail page.
I use Compuserve email on my MAC, and I get a lot of SPAM. Sometimes (maybe once every couple of weeks) when I wake the MAC, it will start beeping kinda slowly and at the same time I'll hear the wooshing that it normally makes when emails are sent. It does this all by itself and usually lasts 10 or 15 seconds and maybe here 10 whooshes. Do you think it might be an infected email, sending out SPAM? When this happens, there are no new emails in "sent mail" that show me that I sent something.
Thanks, Terry P