A few users testing our Citrix XenApp service found that several (seemingly random) previously sent emails were sent out immediately after they logged into Outlook via Citrix for the first time.
The problem hasn't occurred for them since. After one user had this happen, and we scratched our heads about it thinking it was a fluke, our IT director had this same issue. I guess I'd rule out any PEBKAC issues.
I really don't know where to begin troubleshooting this problem. If you have any ideas what could have caused this, I'd appreciate you sharing them, as strange or far-fetched as they may seem :)