Help please

Hello i have the following problem with a client of mine.

Today i installed a adsl line and a pix 501 firewall. They used to have a isdn dialup connection.

Everything seems to be working perfectly, but they are unable to recive mail via the exchangeserver 5.5 and NT. Sending mail works fine.

What could be the problem? And how do i fix it.
Is the problem with the exchange 5.5 og with the pix blocking all incoming traffic?