“You can set up MobileMe to use any email address as a ‘push’ email on the iPhone. The first thing you need to do is set your non-MobileMe (.Mac) email address for auto-forward to your MobileMe (.Mac) account,” dallas2xist reports for macosxhints.
“When you create the email account, make sure you set it up as Other (not as a .Mac or MobileMe account.) Then select IMAP as the type of account. Enter the name and auto-forwarded email address,” dallas2xist reports. “(This is the email address that will show as the “From” email, even though you’re using your MobileMe account.)”
dallas2xist reports, “For your mail server settings, use your MobileMe settings [and you’ll be] able to receive ‘push’ email from your non-MobileMe account, and reply without using the @me.com address. (You can use those same settings in the Mail App as well.)”
More details in the full article here.