Question : Two IPs on the same dot com

I have mx records that point two different dot coms to two different IP addresses with a priority of five each.  When my primary internet goes down, we still get email because it routes through the backup.

Primary: mail.sample.com
Backup: mail-backup.sample.com

This works, however, all the android, windows phones, and iphones quit getting email because they are pointed to mail.sample.com.

Can I make mail.sample.com resolve to a different address when my primary internet fails?  I thought of round robin but I don't want the backup to be used unless necessary.  It's less then 1/5th the speed of the primary and we use it for cisco phone traffic and 3rd party software through a vpn tunnel.

Thanks,
Jason

Answer : Two IPs on the same dot com

Not in DNS unless you actively change the DNS record.

You would need a hardware load-balancer of some kind I'm afraid.

Chris