Microsoft Upgrades Exchange Online Dynamic Distribution Lists

Update Intended to Make Dynamic Groups More Reliable

Update March 8: Microsoft published MC340293 to announce the feature. It’s odd that they do so some months after people saw the functionality work in their tenants. Microsoft says that they will begin the general roll-out the feature in April 2022.

A LinkedIn post discusses Modern Dynamic Distribution Groups (also known as dynamic distribution lists) and says that Microsoft is going to roll out this upgrade for these very useful objects in January with worldwide completion due in March. The fundamental change is to move away from on-demand resolution of group queries to resolve and store group membership daily.

The reasons cited to make the change are to Reduce mail delivery latency by removing the need to resolve the queries against the Exchange directory each time someone addresses the group. Because they cover a limited set of well-known conditions, it’s usually easy for Exchange to resolve precanned filters to find a set of mail recipients. However, custom filters can be reasonably complex and their resolution can slow the ability of the Exchange transport service to route messages. Microsoft believes that making the change to pre-resolved recipient lists will also Improve service reliability.

According to Microsoft, they released details of the change in message center notification MC289967 in early November. I never saw this notification and no trace of it exists in my tenant. However, everything seems to be working as reported in my targeted release tenant. Microsoft says that full roll out will start in early January and should be available everywhere in March 2022.

Less Dynamic but Still Valid

Of course, creating recipient lists for dynamic distribution groups makes them less dynamic and depends on an assumption that group membership doesn’t change often. In most cases, the assumption is true: organizations do not update directory attributes every few days and the membership of dynamic groups probably doesn’t vary all that often.

A side effect of the change is that when administrators create new dynamic distribution groups or update the filters of existing groups, it can take up to two hours before Exchange Online calculates the membership of the new group and makes it available for use. This is different to previous behavior when Exchange could use a new or updated filter immediately it was updated in the directory. The new Exchange Admin Center already flags the need to wait to administrators (Figure 1). True to its word, when I created a new dynamic distribution group, it was available almost exactly two hours later.

To see when Exchange Online last computed the membership of dynamic distribution lists, run the Get-DynamicDistributionGroup cmdlet and examine the CalculatedMembershipUpdateTime property. The results shown below were obtained at 15:00 on 10 December 2021 at, so membership was approximately 18 hours old at that point.

Get-DynamicDistributionGroup | Format-Table DisplayName, CalculatedMembershipUpdateTime

DisplayName                              CalculatedMembershipUpdateTime
-----------                              ------------------------------
Company-DDG                              09/12/2021 20:50:06
Dublin users                             09/12/2021 20:50:06
Office 365 Gurus                         09/12/2021 20:50:06

Moving away from on-demand membership resolution means that Exchange Online now uses the same approach to calculating dynamic group membership as Azure AD uses for its dynamic groups. No great issues have been reported by organizations struggling to deal with Azure AD group memberships, so there’s no reason to believe that problems will surface with Exchange Online.

New Get-DynamicDistributionGroupMember Cmdlet

Because Exchange Online now calculates the membership of dynamic distribution groups daily, the membership of these groups is available without having to run the Get-Recipient cmdlet to resolve the group query. For example, instead of fetching the recipient filter from the group and using it as an input to the Get-Recipient cmdlet like this:

Get-Recipient -RecipientPreviewFilter (Get-DynamicDistributionGroup -Identity Architects).RecipientFilter

Name           RecipientType
----           -------------
Ben.James      UserMailbox
Brian Weakliam UserMailbox
James.Joyce    UserMailbox
Marc Vilas     UserMailbox
TRedmond       UserMailbox
Vasil.Michev   UserMailbox

You can use the Get-DynamicDistributionGroupMember cmdlet instead:

Get-DynamicDistributionGroupMember -Identity Architects

Name           RecipientType
----           -------------
TRedmond       UserMailbox
Marc Vilas     UserMailbox
Vasil.Michev   UserMailbox
Brian Weakliam UserMailbox
Ben.James      UserMailbox
James.Joyce    UserMailbox

Although they’re not returned in the same order, the two cmdlets produce the same set of mail-enabled recipients.

The Get-DynamicDistributionGroupMember cmdlet is available in the Exchange Online management module (and based on Microsoft documentation, the cmdlet has been around since mid-August 2021, even if no one noticed). However, no email client yet supports the ability to:

• Expand the membership of a dynamic group and insert each member as a separate recipient in the header of a message.
• View the membership of a dynamic group through the address book.

As of November 2022, no Outlook client supports these features.

Trivia note: The Microsoft Exchange PowerShell Cookbook, published in 2015, includes a function called Get-DynamicDistributionGroupMember which uses Get-Recipient to resolve a recipient filter against the directory.

Not Much Impact

Only Microsoft can say if the change to precalculated membership speeds message delivery and increases the robustness of the Exchange Online transport service. It’s easy to see how performance will improve, but you’d assume that Exchange had mastered the resolution of membership queries by now (dynamic groups first appeared in Exchange 2003). In any case, the change won’t make any difference to end users. Some might notice if Microsoft updates Outlook to support membership expansion of dynamic groups, but others will never see a difference.

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