Multiple "MX" records with priority must be added to the DNS to begin using mail.
Certain ports must be open for the mail server to send or receive emails.
There are other things too - SMTP, like virus scan, spam check etc..
Also, it is advised to not host the mail server and website on the same server - if the server goes down, both go down.
I suggest you use free ZohoMail, to begin with.