The mail server is the pathway through which all mail pass. If you send a mail, then it will go through a mail server, and if you receive one, then also it will come through a mail server.
Mail servers are highly important as they make the whole process of sending emails and receiving emails highly convenient. When you enter the button to send the mail, it is sent to the server; then, it sends it to the receiver.
In the corporate world, different serves are sued for a different platform to share Emails with each other. There are different types of mail servers, such as Linux mail server, Windows mail server, etc.