What is a mail server?
Mail servers handle email requests for both send and recieve email.

When you send email, it goes to your ISP’s mail server and it’s relayed to your destination’s mail server, then relayed to the recipient you addressed it to.

Why have a mail server?
Having an in-house mail server allows you to have total control over your company’s email. Buying this service from an ISP puts your ISP in control over your company’s mail and they can’t customize the service for one client, as many are using the same server. You can create as many email accounts as you wish with no added costs. Also, you can scan for viruses on the email server and use it a ‘filter’ to filter out junk mail before it gets to the person in the office network.