When you host a secondary MX server and the primary SMTP is out of order for more than 5 days, you will start to loose e-mail from the MX backup. There is a (temporary) easy way to deal with this: raise the retention time of the Postfix mail queue.
maximal_queue_lifetime = 30d
and reload Postfix. You’re done!
Keep an eye on the mail queue while the primary MX is down. Just to be sure not to have the secondary explode…