Modern dynamic distribution groups in Exchange Online

Dynamic distribution groups (DDGs) in Exchange Online are modernized to bring a more reliable, predictable, and better performing experience. This change reduces mail delivery latency, improve service reliability, and allow you to see the members of a DDG before sending a message.

The membership list is now stored for each DDG and is updated once every 24 hours. You know exactly to whom the message is being sent, and it also addresses potential compliance issues. Storing the calculated list of members on the DDG object allows messages to be delivered more quickly and gives our service has greater reliability.

DDG behavior changes

The changes in behavior for dynamic distribution groups in Exchange Online are described in the following table:

Area	Old behavior	New behavior
Mail delivery latency	Unpredictable. The time it takes to deliver mail to a DDG depends on how complex the filters are on that DDG.	Faster and more predictable overall. You should see delivery times more in line with regular distribution groups.
Creation	DDGs could be used immediately after being created.	It takes 2 hours for the initial membership list to be calculated and be available for use.
Modification	DDGs could be used immediately after any changes were made	Users have to wait up to 2 hours for the membership list to be recalculated and links updated.
Membership list "freshness"	The list of members was up to date in real time.	The list of members for each DDG is refreshed every 24 hours.

Important

The list of DDG members might become stale. For example, a user leaves a department used as a filter for the DDG. They might continue to receive mail sent to the DDG for the next 24 hours util the membership list is refreshed.

Mail flow rules (also known as transport rules) are also affected, rules use the membership list that's also refreshed once every 24 hours.

View DDG members

To view the members of a DDG, replace <DDGIdentity> with the name, alias, or email address of the DDG and run the following command in Exchange Online PowerShell. The command returns the calculated list of members stored on the dynamic distribution group object.

Get-DynamicDistributionGroupMember -Identity <DDGIdentity>

For detailed parameter and syntax information, see Get-DynamicDistributionGroupMember.

Note

Don't use the old procedure for viewing members of a DDG as described in View members of a dynamic distribution group in Exchange Online. The old procedure returns all users that satisfy the DDG filters at the time you run the command. The old procedure doesn't return the calculated list of members that are stored on the DDG object.

Refresh the membership of a DDG

If your DDG membership list isn't updated after the next 24-hour refresh interval, you can force a membership refresh in Exchange Online PowerShell. Replace <DDGIdentity> with the name, alias, or email address of the DDG and run the following command:

Set-DynamicDistributionGroup -Identity <DDGIdentity> -ForceMembershipRefresh

For detailed syntax and parameter information, see Set-DynamicDistributionGroup

Note

You can run the refresh command only one hour after the last membership refresh.