I've disabled mail server in the control panel, but the services are still running. Can I just disable the services or are they still needed?
By disabling mail server in ispconfig does not turn off system services, that's correct. What do you want to disable in particular? It depends. When you stop Dovecot/Courier it should be OK, if you do not use it. But if you stop Postfix, it may cause that your emails from websites (if you are using webserver) stop sending using PHP mail() function. Or any other system emails send to root for example (if you processing them).
Great, then it is disabled! I don't allow using PHP for sending e-mails anyway, managing the IP reputation is too much work and people just abuse it anyway. Thanks for the quick reply!