Hi,@Glenn Maxwell
We can use PowerShell to check if user1@contoso.com is a member of any DL (local or online).
The command you provided checks whether user1@contoso.com is a member of the onpremDL1@contoso.com group.
You can run the following command to retrieve all DLs that contain user1@contoso.com.
Get-DistributionGroup | where { (Get-DistributionGroupMember $_.Name | foreach {$_.PrimarySmtpAddress}) -contains "user1@contoso.com" }
Here are the results of my tests:
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.