How to delete mail on server when deleted from trash folder

If you are using POP, that's a bit different.

The idea behind POP is to download from the server, not to keep the mails synchronised between your computer and the server. So with a typical POP account, once the email reaches your computer it's not anymore on the server.

Of course, you can ask to keep the emails on the server, as a copy. But there is just one way with POP, it's from the server to your computer.

If you want to keep a copy of your emails on the server, the best solution is IMAP. IMAP keeps the emails synchronised. So if you pick up your emails, they also stay on the server. If you delete on the server, they will be deleted on your computer at the next sync. If you delete on your computer, they will get deleted on the server.

However, maybe you cannot set IMAP instead of POP with your mailbox, it depends of the provider. Gmail for instance doest not offer IMAP, only POP. If your provider offers IMAP, then I suggest you to backup your emails (just in case), create another account with IMAP protocol, open it (it should download all your emails), then close the POP account. But never forget to make a copy of your emails, you can never be too cautious.

I hope it answers your question.