That's understandable. At the same time, an important email's only copy probably should not be on your iPhone.
Fortunately, you can set up iPhone Mail so that it deletes from the server automatically after downloading — but only messages you've removed from your iPhone's inbox. Other options include keeping copies of all messages on POP servers and removing them a week after downloading, which should be time enough to download them to a safe and permanent place.
Make iPhone Mail Delete Mail from POP Servers (or Keep It)
To have iPhone Mail remove downloaded messages from your POP account automatically:
- Tap Settings on the iPhone Home screen.
- Now tap Mail, Contacts, Calendars.
- Tap the desired account under Accounts.
- Select Advanced.
- Now tap Delete from server under Incoming Settings.
- Seven days to have iPhone Mail remove mail from the server a week after it has been downloaded.
- When removed from Inbox to make iPhone Mail delete mail from the server only when you delete it from your iPhone Mail inbox.
- Never to deep copies of all messages on your POP server.
- Tap Advanced to return to the account's settings.
- Press the Home button.
(Updated September 2012)