Dynamic distribution group not updating
What you consider to be “large” might be different than what others do, but by default a large audience is 25 or more recipients.This threshold is defined in the Exchange organization config.Since Mail Tips warn about a variety of conditions, including Out of Office for recipients, and restricted distribution groups, they are quite useful.In particular, the “large audience” warning is useful in reducing the likelihood that someone will send an inappropriate email to a large number of people.Therefore for mailboxes in the Office 365 cloud they will not see the Dynamic Distribution Group in their Global Address List, and so therefore can only email the members of the list by sending an email directly to their email address.To show the Dynamic Distribution Group in the GAL in the cloud, you need to add a Mail Contact to the cloud that represents the Dynamic Distribution Group.
The file can be found in the Group Metrics folder of the Exchange install directory.The behavior of the large audience Mail Tip is slightly different though, if one single distribution group exceeds the threshold.Instead of seeing a warning about the total number of recipients, you'll see a warning about the size of the large group.Displaying members of all dynamic distribution groups is not much harder.The trick is to display them in an orderly way instead of lumping them all together.
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Group metrics generation happens automatically as an ongoing background process, but if your servers are overloaded then group metrics may not be updated regularly (or at all).