Symptoms
When you create a transport rule (mail flow rule) and select the SentToMemberOf (is member of this group condition) parameter, it should show only the distribution groups, dynamic distribution groups, or mail-enabled security groups. However, the list shows everything including normal mailboxes, contacts, linked mailboxes, and every recipient type.
Cause
The filter that’s used in this scenario isn't correct.
Resolution
To fix this issue, install one of the following updates:
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The Cumulative Update 3 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
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The Cumulative Update 14 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
References
Learn about the terminology that Microsoft uses to describe software updates.
