I don't think the stock gmail app, if that's what the OP is using, has that option (which would make sense, since the gmail app is using an IMAP style system).
The problem is that when you delete your email on your phone, you're also deleting it from the server. That's compared to your PC, where if you delete the email in outlook, the email is still on your gmail account. The easiest solution would be to simply start hitting 'archive' on the gmail app rather than delete. It won't use up any more space on your phone and you can still find the email in outlook later.
But if POP is what suits you, then K9 is a good app, definitely.