How can I delete multiple e-mails (say from the trash file) without crashing my Sony Xperia Z Ultra?

[AC Question]

I have seen the followin issue discussed elsewhere: when an inbox email is deleted, it moves to trash and then must be deleted again to free up memory. As a result of running three inboxes on my Ultra, I have 8000+ email sitting in inbox and trash box and I haven't even bothered to figure out the sent box. Any advice will be greatly appreciated.