Disasters can take many forms and, by their very nature, are unexpected. If DNS and mail are to continue to work, expecting the unexpected is vital. The kinds of disasters that one must anticipate vary from the mundane to the catastrophic:
A reboot or scheduled downtime for maintenance on the mail or DNS server should only cause mail to be delayed, not lost.
A failed component on the mail or DNS server could cause mail delivery to be delayed anywhere from a few hours to a few days. A delay of more than three to five days could cause many hosts to bounce queued mail unless steps are taken to receive that mail elsewhere.
Natural disasters can disrupt site or network connectivity for weeks. The Loma Prieta earthquake on the West Coast of the United States lasted only a few minutes but knocked out electric power to many areas for far longer. Fear of gas leaks prevented repowering many buildings for up to two weeks. A hurricane, flood, fire, or even an errant backhoe could knock out your institution for weeks.
When mail can’t be received, whether because of a small event or a large disaster, an offsite MX host can save the day. An offsite MX host is simply another machine that can receive mail for your site when your site is unavailable. The location of the offsite machine depends on your situation. For a subdomain at one end of a microwave link, having an offsite host on the other side of the microwave might be sufficient. For a large site, such ...