Sending systems will automatically back off and retry at intervals (I have seen this happen when I have upgraded my home server in the past) so will a secondary/backup MX actually help at all?
It's up to you to decide what your priorities are. It's true that sending systems automatically retry delivering emails, but this means at least (1) that you experience delivery delays when your main MX is down, (2) that if your main MX is down for a long period (e.g. you're on vacation and cannot reboot, your internet connection is physically down and needs to be repaired, ...), and (3) some sending systems (typically mailing lists) maintain a record of delivery failures and will stop trying to send emails for you when there are too many failures. In cases (2) and (3) you might lose emails.
Gregory
