I have a 3Gs jailbroken and unlocked using a T-mobile account without any data plan. I also have SBsettings and when ever I look to check what processes are using memory I see "mail" on the list. I have never set up a e-mail account with my phone and have all the notifications for mail turned off.
Yeah i wouldn't worry too much about that...im pretty sure it's not actually pulling data as long as you have email setup to manually get email and not fetch or push...just close it..