Unable to receive email

I have installed a brand new computer, winXP pro, connected with my existing wi-fi. It goes perfetly on line with browsers, and I can send emails. But for some reason I can’t “receive” emails, it just stalls until the mailer gets tired. I’ve tried with Thunderbird, Outlook and Act, all with the same result. I’ve checked the mail server settings ten times, they are right and exactly like in my other computer which works perfectly. Btw, this make me think also it’s not the router. I use Agnitum Outpost on both of them as firewall, and they are both set the same.

What can it be?