Weird Internet & Mail Problems

Okay, here are the symptoms:
1) My 20" iMac will connect with my FIOS WiFi network, but I can't connect to the internet.
2) All my other wireless macs will connect to network AND internet via WiFi
3) If I connect my PowerBook and iMac with ethernet and share my WiFi connection from PB to iMac, I can connect to the internet with the iMac, BUT
4) I cannot access my .mac esmail with Mail (it simply won't connect to the servers) AND even weirder,
5) I cannot log in to .mac mail in Safari on the iMac, even though I can log in on the PowerBook

What I've tried that hasn't worked:
restarting FIOS router
running all the normal maintenance routines via Cocktail,
running DiskWarrior (twice, actually)
restarting a couple of times.
resetting my Airport network from scratch.

Any ideas?
(Seems to me that Mail has some setting screwed up, but I haven't a clue what nor how to fix it)

Thanks, Kirk