Having a backup mail server that can receive messages if the primary server is unavailable sounds like a really good idea, but today's reliable Internet connections together with spam, worms, and other rubbish have for the most part made backup mail servers unnecessary and often even harmful. The rationale for having a backup server is that it can receive messages while your primary server is down, and then deliver them to the primary server when it is up again. However, the advantage of this is very small, as all SMTP servers are required to queue undeliverable messages for at least five days before they are returned to the sender. Granted, by having a backup server it is possible to store unavailable messages for longer time than five days. However, if the main SMTP server is unavailable for longer than five days at a stretch then there are probably bigger problems than a few lost messages.

Because a backup mail server typically does not have the same spam-thwarting configuration as the primary server, spammers often specifically target backup servers in order to bypass the stricter rules of the primary server.

Another strong reason to avoid backup mail servers is that they typically do not perform recipient validation. This means that they do not know which recipient addresses are valid for the domains they act as backup servers for. This requires a backup server to accept all messages for the backed-up domains and attempt to deliver them to the primary server. The primary server will reject invalid recipients, causing the backup server to bounce such message back to the sender. This is known as backscatter and is bad for two reasons:

A busy server that does not perform recipient validation and is hit heavily with spam may have thousands or tens of thousands of undeliverable messages residing in the queue.