Well in my opinion i would see with yum list exim witch package is installed and removed it with yum remove. If you installed postfix you should be able to start it with /etc/init.d/postfix start and add it to auto-start on boot with command from tutorial chkconfig --levels 235 postfix on
Also don't know about exim but if it is deamon runing try turning it off with same way as you turn on postfix, so check for exim in /etc/init.d/ and see what you come up with?
If it's there then u can run /etc/init.d/exim stop and then after chkconfig --levels 235 exim off
Thing is i don't have exim installed so all this is based on some preassumptions.
Hope it helps