MX Entry

MX (mail exchanger) entries serve as virtual traffic directors, directing incoming mail to the appropriate server.

Using MX entries, you can assign each mail server a priority. MX entries that are assigned lower priority values are the first to receive a delivery attempt. For example, an MX entry with a value of 0 will function as the your primary mail server — where a delivery attempt will be made first — while an entry with a value of 10 would function as a secondary mail server.

If you assign multiple mail servers the same priority, then when that level of mail server is needed, mail will be distributed to those servers randomly.

Additional MX Entries

If possible, its a good idea to provide multiple MX entries, and consequently mail servers, to function as fail-over and backup systems. A fail-over system allows you to receive mail even though your primary mail server is not functional. As delivery attempts to the primary mail server fail, the secondary mail server will be tried.

Once the secondary mail server receives mail, it will hold the mail in queue and send it to the primary mail server once it comes back online.