Check out the MX record for DNS. You can set up both to receive and send mail and Internet sites sending to you should use your MX record to get the list of mail exchangers.

If you do a nslookup against yahoo.com:
> yahoo.com
Server: which.imedia.com
Address: 10.140.16.20

Non-authoritative answer:
yahoo.com preference = 1, mail exchanger = mx1.mail.yahoo.com
yahoo.com preference = 1, mail exchanger = mx2.mail.yahoo.com
yahoo.com preference = 5, mail exchanger = mx4.mail.yahoo.com

Authoritative answers can be found from:
yahoo.com nameserver = NS3.EUROPE.yahoo.com
yahoo.com nameserver = NS5.DCX.yahoo.com
yahoo.com nameserver = NS4.DAL.yahoo.com
yahoo.com nameserver = NS2.SAN.yahoo.com
yahoo.com nameserver = NS1.snv.yahoo.com
NS3.EUROPE.yahoo.com internet address = 217.12.4.71
NS5.DCX.yahoo.com internet address = 216.32.74.10
NS4.DAL.yahoo.com internet address = 63.250.206.50
NS2.SAN.yahoo.com internet address = 209.132.1.29
NS1.snv.yahoo.com internet address = 216.115.108.33

If you do it on unix.com
> unix.com
Server: which.imedia.com
Address: 10.130.16.20

unix.com preference = 0, mail exchanger = unix.com
unix.com nameserver = ns.silkroad.com
unix.com internet address = 64.23.37.23
ns.silkroad.com internet address = 64.23.37.23

On Yahoo, if one is down, the rest will take care of the email. On unix.com, if the one is down, there is no backup (oops!)