There is absolutely no problems to run the mail server in server only mode. I have used it as in/out mail server, fileserver, domaincontroller at work and at home and allways configured as server only. (Because it fit best with the way my network have been buildt, there were not need for any aditional roter.)
I think that things lik ip, dns server, gateway etc is among the things you are configuring during the installation.
If you did not, as far as I remember .. you can log on to the sme server via shell as root and get en ordinary Linux logon. Alternatively you can log on using the admin acount and you will jump right into the menues where you do the basic configuration, ip adress, gateway, dns server and so on.
At least it work like this on my sme servers, but there is some alternative choices during installation, so there might be some differences. Don't know about that for sure, as I normally does it the same way.
To log in via putty.exe as admin and make a new "basic configuration" I believe is the medicine I have used when things have come far out.
Problem: The sme servers runs for years without problems, so it is difficult to remember.
Arne.