MAIL
Using a single Internet domain with a single domain name
You can specify MX records for a single Internet domain -- for example, acme.com -- with a single Internet domain name, such as acme.com. Use the server's fully-qualified Internet host name in the MX and A records -- for example, mail1.acme.com.
For example, configure a backup SMTP server (mail2.acme.com) to deliver or forward mail when the primary SMTP server (mail1.acme.com) is unavailable:
1. MX record: acme.com IN MX 5 mail1.acme.com
Messages addressed to acme.com route to mail1.acme.com first because the record's preference (5) is lower. If mail1.acme.com is unavailable, mail is routed to mail2.acme.com.
If you specify equal preference for two servers, DNS randomly selects a server to balance the load of incoming mail.
You can create MX records for a single Internet domain -- for example, acme.com -- with multiple Internet domain names -- for example, acme.com, qrs.com, and xyz.com.
Note Users can address mail to each domain name and each domain has a backup SMTP server.
2. MX record: acme.com IN MX 10 mail2.acme.com
3. MX record: qrs.com IN MX 5 mail1.acme.com
4. MX record: qrs.com IN MX 10 mail2.acme.com
5. MX record: qrs.com IN MX 5 mail1.acme.com
6. MX record: xyz.com IN MX 10 mail2.acme.com