You have an on-premises Exchange organization. The organization plans to migrate to Exchange Online.
Users report that after their mailboxes are migrated to Exchange Online they are no longer able to send email to a specific dynamic distribution list. All other distribution lists work as expected.
You need to resolve the issue.
What should you do? environment, certain members of a dynamic distribution group do not receive email messages. deployment and if the dynamic distribution group uses filters to include only mailboxes. Mailboxes that are migrated to Office 365 become mail-enabled users in the on-premises directory. mail-enabled users.

A.
In the Active Directory Synchronization Services console, change the connector filter to include dynamic
distribution lists.

B.
In Office 365, re-create the dynamic distribution list.

C.
Run the following Windows PowerShell cmdlet: Set-DynamicDistributionGroup

D.
Reduce the number of members in the distribution list to fewer than 1,500 contacts.

Explanation:

PROBLEM
You have a hybrid deployment of Exchange Online in Office 365 and on-premises Exchange Server. In this
CAUSE
This problem occurs if the dynamic distribution group was set up before the environment became a hybrid
SOLUTION
Use the Set-DynamicDistributionGroup cmdlet to update the filters for the dynamic distribution group to include
References: https://support.microsoft.com/en-us/kb/3061396