Have had several reports (myself included) since moving mail services onto this new server of users downloading mail, having one message appear in their inbox, but when looking at the body of the message, it clearly should have been 2 or 3 separate emails. I'm not versed enough to determine exactly which program is messing up... but have tested with 3 separate mail clients (Outlook 2000, Outlook Express and Pine) and all 3 show the messages corrupted in the same way, so I'm assuming that the mail delivery agent is failing to properly separate/terminate the messages when it is storing them in the local mailbox.
There's not enough information for me to even attempt to reproduce the problem. If you can supply enough information to generate a reproducible test case, please reopen.
Resolved the problem myself by removing the RPM'd sendmail and downloading and compiling my own copy... glad I didn't wait, 7 months is an awful long time.