For some time, I wanted to find out how ISPs set up a backup mail server for serving many corporate accounts together. The logic is that if a company has its primary server failed, the incoming email will be queued up at the backup mail sever of an ISP by means of the following typical MX records:
xyz.com IN MX 10 mail.xyz.com.
xyz.com IN MX 20 relay.isp.com.
The trick is how to configure relay.isp.com as a backup mail server . It is really not too difficult if one has a fairly good background of mail relay. Just configure an ordinary Sendmail Server which acts as a relay host for the email destined to domain xyz.com in /etc/access. Since when the primary server ecncounters a failure, it might take several hours to resume to normal. The retry interval of the queued mail shall be set longer, say several hours.
I have tried to use two mail servers one backing up the other and the tricks above really work.
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