Lets look at this from the perspective of a server sending mail to your domain.
First i need to find your mail servers, so i look up MX records for your domain. You may have these:
example.com. 3600 IN MX 10 mail.example.com.
example.com. 3600 IN MX 20 aspmx.googlemail.com.
What does this mean? Domain example.com has 2 MX records which point at different mail servers. The first number for each record is the TTL, which determines how long it can be stored before i have to look it up again. the second number is the MX priority which determines the order I should check in with the mail servers. The number is arbitrary, here i use 10 and 20. One is less than the other.
The first mail server I should try is mail.example.com so I then do a DNS lookup for mail.example.com
example.com. 2457 IN A 18.104.22.168
I see that mail.example.com has IP 22.214.171.124 and i try to deliver mail there. If i can't reach it i try the second server which was aspmx.googlemail.com., and then so on until i run out of servers. If i can't deliver the mail at that point i would send the message back to the sender.
So the MX records for example.com tell me which domain names have the mail servers for example.com. Each mail server would have to have its own SSL certificate for its domain name. If random.example.com was set as a mail server in an MX record then random.example.com would have to have its own SSL certificate for that domain name.
You generally only have one server for retrieving mail. It is the server that keeps a canonical copy of your mailbox on it. It can have any domain name; you would put this domain name in your mail client.
I would advise against running a mail server on your home internet connection. If your connection or server goes down you will not get mail. Your connection will be filled with spam hitting your server at an incredible rate. Its a fun experiment but i would not rely on it. Instead i recommend either paying for a mail service like fast mail or using the free edition of google apps for your domain.