This is a very minor cosmetic issue, hence the declaration as "task", but I'd still like to mention it.
Clicking "Select all" in Edit mail and FTP users also turns on the checkbox for the primary domain user, whose checkbox is actually disabled.
That user is correctly skipped when doing the actual "Delete selected users", still it might possibly cause a slight confusion or raised "uhm, you don't really mean that?" eyebrow. :)