I have scowered Google for the past few months, but have not found any clear answers, that describe the PRE-setup to email servers. I understand that I need my own domain name first, which is not a problem. I understand that I need an MX record to be forwarded to my server, which I assume is in DNS when I sign up for the domain. Let's say for example that I have Shaw for my Internet, and a computer that meets the minimum requirements to run a Linux based mail server. Now, my question is:

1) What do I need to ask for from my ISP to make sure email will come to my server?

2) What DNS setup needs to be done?

3) If I am going to host multiple email domains, do I need separate lines or separate software? Or just change a port number?