The rule of thumb is if you are not using a service, turn it off. So if your not using telnet, nfs, etc then go ahead and turn them off. The easiest way to do this in fedora is with the chkconfig utility and service command:
To list all the services and their status (turned on or off in each runlevel):
chkconfig --list
From there, turn off the services at boot:
chkconfig
<servicename> off
Then stop the currently running service with:
service
<servicename> stop
so to turn off rpcbind (portmap):
chkconfig portmap off
service portmap stop
Having smtp (mail) running isn't bad per se, but should be listening for localhost only rather than external traffic. With Fedora core that is usually the default configuration, so I'm guessing you ran nmap from the same machine you were scanning. Ideally you should be running scans from the outside (a different machine) to get an idea of what an attacker would see. So you can leave smtp on as long as it is listening for localhost. To confirm this, run netstat -pantu and you should see something like this:
Code:
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 568/sendmail: accep
The important part is that sendmail is listening for 127.0.0.1
For info on general secuting of a linux system, take a look at unSpawn's
Security References Thread.
If you want to monitor network traffic over time and get a good idea of the network topology, etherape is a really nice tool that is pretty powerfull.