In /etc/postfix/main.cf I have [relayhost = smtp.someisp.no] and everything works fine. I have to use this relayhost since my ISP has closed port 25. My ISP tells me that if I dont want to use their relayhost I have to configure my mailserver to use a ssl port like port 465. Then I can remove the relayhost option. Do anyone know where I do this like which files to edit and is this possible if I dont set up my server as a DNS Server?
It sounds like you're talking about two different things.
If you need to configure a
My guess as to what the person at the help desk was suggesting was that if you wanted to contact a particular mail server outside the ISP's firewalls, then that mail server should be configured to listen on a second port. Some reasons you might want to do this is if your company required that all mail be sent via their servers, or you'd set up restrictive SPF rules such that mail sent via your ISP would be rejected. One thing to keep in mind here though, is that this would just be trading one relay host for another.
If you don't have such an external mail server in mind, then the suggestion about changing the port doesn't really apply. In that case, relaying via your ISP's mail server is probably the best option.