it depends on the kind of control you want to have on your mail server.
You can choose between the easy way where you place your mail server on a hosting service or the hardest way where you keep your mail server in house.
The first approach is faster and you only need to identify the best offer to you for hosting solution, then after you have all the things done you can configure any mails erver (Example Microsoft Exchange) and use the POP3 connectors to download messaegs from the hosted domain to the local users ; the second approach instead involves you asking for IP+DNS domain registration and correct MX records setup, later than you have to simply configure your mail server to use this data and you can start.
The main difference between the two approaches is that in the first solution you don’t have to worry about 24/7/365 or anything, also many hosts provide a good spam fighting technology. With the second approach instead you have to care about everythin but also you have full control on what happens on your mail server (you can itnegrate it with other products, upgrade it and so on).
I hope this helps.