The problem occurs with Hotmail no matter which computer I use to check the mail. I sent an email at 6:28 PM and checked hotmail a few seconds later. Hotmail indicates that the mail was sent at 3:28 PM.
Have you looked around at all the config options on the hotmail page?
Is there some sort of time offset available there?
I have a hotmail account, but rarely send an email from it. However, it does stamp the time correctly.
I don't remember if I ever had to set the time offset in that hotmail account - I've had it too long now.