In a Microsoft Exchange Server 2019 or Exchange Server 2016 environment, assume that you have a large distribution group that contains many users. When the users are in different domains (for example, some users are in the domain where the distribution group locates in, and the other users are in a different domain), you may experience that the MailTip shows an incorrect count of the group members.
This issue occurs because Exchange Server just counts the group members who are using the same domain as the distribution group.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
To fix this issue, install one of the following updates:For Exchange Server 2019, install the Cumulative Update 1 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.