If you can not delete a mail with no addon enabled, the bug must be in build-in script.
After deleting a mail the EVENT_MAIL_INBOX_UPDATE event is triggered.
One of the registered handlers updates the in-box (Suprising
)
Another handler updates the list of recent interactions from the mails left in the box.
But being in Cyrodiil you have a lot other interactions too, I guess.
Maybe there are names breaking the system?
The list of recent interactions is saved on /reloadui. This may causes the crash.
But if this is true, the crash must be gone once the game is restarted. Except this special name is near to you or in your in-box?
Just guessing.