They will only receive your domain’s emails when your box is down.
Using backup MX fell out of favor when spammers (and just plain disruptive/malicious people) started detecting that a backup MX was in use and hit the domain especially hard, which tends to cause an excess of email to the domain which in turn overwhelms the original server when it is back up. So, kind of an attack.
Honestly, if you lost email during a short downtime (yes, even 6 hours) then the truth of the matter is that you must not have set up Secondary DNS. The sending email servers should be retrying the failed emails for several hours/days. (The norm is 2-3 days).
However, they can only retry mail if there is a DNS record. The way MiaB is set up, you must take extra steps to set up Secondary DNS.
This article I wrote goes into more detail: