MX (mail exchanger) resource record identifies the mail server that is responsible for handling e-mails for a given domain name. When more than one MX record is entered for any single domain name that is using more than one mail server, the MX record can be prioritized with a preference number that indicates the order in which the mail servers should be used. This enables the use of primary and backup mail servers, which makes mail delivery more reliable. Please, keep in mind the following rules about MX records:
MX record must point to hosts defined by A records, not CNAMEs.
MX record cannot point to IP addresses.
If you create a single MX record, the priority doesn't matter. If you create multiple MX records, your primary mail server must have the lowest preference.