Ok, guys, I just had the same problem myself but I figured it out. It's actually very simple.
A friend of mine just bought an iPhone so he gave me his iPod Touch. I love it!
Anyway, the problem lies not in the default mail account but in the default smtp (outgoing mail) server information specified. Regardless of how many mail accounts you have or which one you have set for default, the smtp server you have specified is the server that the iPhone or iPod Touch will use to communicate with to send out your email.
The way you can view/edit this information is:
Go to main page, then:
Settings --> Mail, Contacts, Calenders --> Mail Account (in my case, gmail) --> scroll down and click on 'SMTP' --> Primary Server --> here you can specify any/all settings you need including server, user name, password, etc.
Worked for me!
Good luck...