I was looking at the memory used by my emails and found it was quite a lot already, so I think there's a couple of ways to avoid making the phone unusable for anything else than receiving email :)
- one should be able to select the memory card for storage of emails
- one should be able to remove messages "from the phone" (leave on server)
- one should be able to "remova all messages for this account from the phone" (leave on server)