There seems to be a bug in Acronis True Image 2018 & 2019 that exchange and iMAP emails, OST files are been omitted from the backup.
Acronis seems to blame Microsoft for this fault but either way, here is a solution to resolve the issue.
Open Windows Registry Editor by searching for regedit in the search box
Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\BackupRestore\FilesNotToSnapshot
Delete the OutlookOST subkey (marked in green in the image below)
Now create a new backup and this will contain your exchange or iMAP *.ost file/s
Note: An easy way to make sure you have a new backup containing all files is to follow this guide on to clone your backup job but do be careful and if unsure, do not proceed without the assistance of an IT Specialist