I have two clients who use Macs at the same location. One Mac has OS 10.8 and the other with 10.9 and both have MS Office 2011 for Mac with all the current updates. Both have email setup as POP accounts.
One day while my clients were working on emails, the lights went out. When they rebooted, the email they were both working during the power outage came up where they left off. Those emails are obviously saved in the Drafts folder but even if you delete them from the Drafts folder then delete them from Trash, they still come up again upon restart of Apple Mail (even if you reboot the Mac).