Already set up LAN, now mail not coming through but no errors

I finally got around to setting up a router here in the office and connecting a couple of the other computers.  Basically I am using a SMC Barricade that allows one wireless connection and up to four Cat5 wired connections.  I have been using the router for over a month with no problems (just using the wireless access point and one of the cabled entry points), but as soon I set up two of the other connections, we stopped receiving mail on the main computer (the one already online, using the first cabled entry point).  The problem is that we aren´t receiving any errors on the mail client (Microsoft Outlook).  It just says that there are no new messages on the server.  I have sent numerous messages to the account, and we regularly receive 25-50 messages per day, but we haven´t received anything since Friday, Dec. 5 at noon (exactly when I got everything connected.  Our provider tells us that everything is fine on the server side, and we are able to browse, but no mail.  Does anybody have any ideas?I realize that you guys/gals will probably need more information, so just ask me what you will, and I´ll see if I  can find.

thanks  -  joel