DNS MX Records

DNS MX records are used to report the IP address(s) of the email server(s) for a particular domain.

E.g.
Here we can see the MX record for si.edu:

% dig si.edu MX +short
0 si-edu.mail.protection.outlook.com.
%
We can see the priority is set to 0 and the host for the email server is si-edu.mail.protection.outlook.com. DNS could then look up the A record for that host and it would have the IP address of the email server.