Symptoms
When you do an eDiscovery search in Microsoft Exchange Server 2019, if a mailbox contains lots of email messages that arrived at approximately the same time, the item count in the search estimate differs from the item count in an export to a discovery mailbox or .pst file. This issue occurs because some items are not counted during an export. The search estimate value represents a more accurate count.
Resolution
To fix this issue, install Cumulative Update 12 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
