I wonder if your anti-virus program is locking the mailbox file associated with the Trash because it's now "infected"? Mail is stored locally in mbox format as explained for the server - one large file containing all the e-mails for the particular folder in TB.
There's no point in returning it because you'll be tarring your ISP as an originator of malware (assuming they don't filter it out), the originating address is almost certainly faked, & even if you do manage to find the actual originating computer it's likely they're the victim of malware that's doing the dirty work.