Usually, the backup MX just gathers email (so that the email does not bounce to the sender) and delivers it to the primary MX when it gets online and then the p

Usually, the backup MX just gathers email (so that the email does not bounce to the sender) and delivers it to the primary MX when it gets online and then the primary delivers emails to the server that is going to expose it via IMAP or POP3. But this is just for receiving emails (from the outside world), if your IMAP/POP3 server is on the same server as your primary MX you will not have access to emails on the server while the primary is down. You have to find a way to sync your received mail (maildir/mailbox) to two or more servers. If you decide to implement a backup MX, try to sync your allowed recipients list from the primary because spammers often try to send emails to an MX with a lower priority, and if it accepts all emails for the domain without checking if that mailbox actually exists you could became a source of back-scatter.