Ah, it's very likely that the system is just trying to bounce this type of email since it knows you did not send it. Check your email queue to see if you have emails stuck in there. This is the spammers actual objective is to have the emails bounce to their real target. This is the reason why I stopped using Spamassassin as I could not find where to simply trash these emails rather than bouncing them.
These emails were making it thru to my local inbox and not being rejected by exim nor are/were there any stuck in the queue. But! I found the spambox, as you pointed out and they are trapped there and not being bounced - also <ducking> I was logging into webmail as the acct. name and viewing all my associated emails. I logged into webmail as the user getting the spam and subscribed to the spam folder (which didn't appear using the acct name) and the spambox is present with the spam!
Here's what one of them looked like in the exim log:
2009-06-07 15:00:44 1MDPUF-0005m8-R5 H=ppp-124-121-149-181.revip2.asianet.co.th (ppp-124-122-6-95.revip2.asianet.co.th) [126.96.36.199]:2361 I=[xx.xxx.xx.xx]:25 Warning: "SpamAssassin as XYZ detected message as spam (16.0)"
2009-06-07 15:00:44 1MDPUF-0005m8-R5 <= firstname.lastname@example.org
H=ppp-124-121-149-181.revip2.asianet.co.th (ppp-124-122-6-95.revip2.asianet.co.th) [188.8.131.52]:2361 I=[xx.xxx.xx.xx]:25 P=esmtp S=7995 id=YJCCV66647.B0969D8@ppp-124-122-6-95.revip2.asianet.co.th T="Douglas asked to send it" from <email@example.com> for firstname.lastname@example.org
2009-06-07 15:00:44 cwd=/var/spool/exim 3 args: /usr/sbin/exim -Mc 1MDPUF-0005m8-R5
2009-06-07 15:00:44 1MDPUF-0005m8-R5 => me <email@example.com> F=<firstname.lastname@example.org> R=virtual_user_spam T=virtual_userdelivery_spam S=8094
2009-06-07 15:00:44 1MDPUF-0005m8-R5 Completed
2009-06-07 15:00:45 SMTP connection from ppp-124-121-149-181.revip2.asianet.co.th (ppp-124-122-6-95.revip2.asianet.co.th) [184.108.40.206]:2361 I=[xx.xxx.xx.xx]:25 closed by QUIT