This has been asked and answered before, though for the life of me I can't locate the thread. It's happened to me twice, and prior to reading that thread I can't find, I performed a new EA as well. The EASY way is;
From Desktop Manager, with device attached via USB or BT, go into Backup and Restore, click advanced, on the right side of the window (device databases,) scroll down to messages. Highlight "messages", and click clear. This should remove the orphaned email message(s) that can't be deleted via the handheld.