It’s important to know if a tenant has any very large distribution lists as these might be the source of reply-all mail storms. An old article explains how to report the membership counts for distribution lists on an on-premises Exchange server. Life is different in the cloud, and we need to take a different approach. This article explains how to use different calls in a PowerShell script to create a nice report about distribution list memberships.
https://practical365.com/getting-exchange-online-distribution-list-membership-counts-with-powershell...
