Any effort to delete mail that was sent before the LoR expansion update is fruitless.
Mail sent after update is deletable, but clicking on Delete button for prior mail has no result... not even a message.
EDIT: Apparently, this is a known issue and is being looked into by devs. Thx.
http://sto-forum.perfectworld.com/showpost.php?p=10119051&postcount=2