With GMail or with something else?
I'm not at my Mac right now, and so I'm being lazy and suggesting this but not bothering to test it, but I think this might be an idiosyncracy in the way GMail is set up... I remember vaguely running into this with GMail also.
OP: Do you have GMail set up as POP or IMAP on your Mac? If it's POP, try iMac... I think the idiosyncratic behavior may have to do with the way GMail implements POP (which is different from the way most everyone else does it), whereas they implement IMAP more similarly to how others do it.