from zonealarm helpfiles.
Server permission allows a program on your computer to "listen" for connection requests from other computers, in effect giving those computers the power to initiate communications with yours. This is distinct from access permission, which allows a program to initiate a communications session with another computer.
Several common types of applications, such as chat programs, e-mail clients, and Internet Call Waiting programs, may need server permission to operate properly. Grant server permission only to programs you're sure you trust, and that require it in order to work.
If possible, avoid granting a program server permission for the Internet Zone. If you need to accept incoming connections from only a small number of machines, add those machines to the Trusted Zone, and then allow the program server permission for the Trusted Zone only.
find outlook express works find if not granted server permission.