MX-records identify mail server responsible for handling mail for your domain name.
When sending an e-mail to "firstname.lastname@example.org", your mail server must first look up the MX-record for "xyz.com" to see which mail server actually handles mail for "xyz.com" (this could be "mail.xyz.com" - or someone else's mail server like "mail.isp.com"). Then it looks up the A-record for the mail server to connect to its IP-address.
An MX-record has a "Preference" number indicating the order in which the mail server should be used (Only relevant when multiple MX-records are defined for the same domain name). Mail servers will attempt to deliver mail to the server with the lowest preference number first, and if unsuccessful continue with the next lowest and so on.
An MX-record identifies the name of a mail server server - not the IP-address. Because of this, it is important that an A-record for the referenced mail server exists (not necessarily on your server, but wherever it belongs), otherwise there may not be any way to find that mail server and communicate with it.
Do not point an MX record to a CNAME-record. Many e-mail servers don't handle this. Add another A-record instead.