From what I understand, it's not possible to do what you're asking. Once you set up your email account to be used on your iPad, it is set up to both send and receive emails, and doesn't give the option to turn off one or the other.
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A306 Safari/6531.22.7)
Why would this matter to you? With MobileMe your email is always in sync regardless of which device you use.