Problem with mail's delivery in XP

[P]

I use OUTLOOK 2000 (SP-3) on Windows XP. When I have a switched on firewall
I receive (or send) automatically mails only once (from or to Exchange
server). Next sending or receiving goes after changing subfolder in Outlook
(it doesn't matter which ones). Pressing the Receive/Send button has no
effect.

If firewall is switched off all delivery processes go automatically (with
correct reaction on the Receive/Send button). Do you have any ideas ?

Magik