You did it through the setup wizard on the phone or you did it on Tmob BIS site?
Do you wanna try delete the comcast email service from BIS and redo it again? I setup a comcast.net a/c for a friend on the net, she didnt get any email for the first 2 days "instantly", about 2 days later till now, she gets all her emails pushed to her phone instantly.
Same as for my phone, I have gmail and yahoo. My yahoo email always delay for a few hours for the first few days, and now I am getting instant push.
For some reason, I think the server has some attitude problem, you need to give it some times to get use to your email.