iPhone Mail, Send only account
I have found the solution!

I set up a second email account on my server with an obscure name, then, I changed the POP/IMAP settings to use that account for receiving, and left the send/SMTP details unchanged.

This now gives me the desired behaviour. It also gives me a receive only account ( which could also be desirable ).

If you are on iOS4 or below, you can also refer to Hand-E-Food's answer. My instructions will work for iOS5 ( due to the extra checks iOS5 makes to verify what you've entered )