Do you have DNSSEC active for the domain your MIAB is in? Then maybe you are running into the same problem I did a while ago.
I have DNSSEC active on the MIAB domain. The MIAB server acts as the mail server for another domain that has its DNS elsewhere. One day some mails to addresses in this domain started to disappear with no errors or bounce messages, while other email to the same addresses, but from other sources, came in fine.
It turned out that some SMTP servers were not able to resolve the address of the MIAB server. I solved this by disabling DNSSEC. After a couple of hours the missing mail started to come in. A day later I re-enabled DNSSEC and since then the problem has not returned. I have no idea as to what caused this, as I changed nothing except switching DNSSEC off and on again.
In my experience email hardly ever just ‘disappears’. It is either delivered to the receiver or returned to sender with an error message. Only this process can take a long time, like a couple of days.