Distribution Group External Senders Restriction

mlaminedouba 

What we need to confirm first is

1. Are the emails from specific external GMAIL users or from all GMAIL users
2. Does the email expands to all members of the Distribution Group or to specific members
3. Can we confirm if this has been working before or a new settings

By design, external members should not be able to email a distribution group after checking the "Only allow messages from people inside my organization".

If per adventure, external members are still getting emails sent to a distribution group after checking the “Only allow messages from people inside my organization” option, it could be due to a delay in the replication of the changes made to the distribution group settings if it has just been saved. It usually takes about 60 minutes for distribution groups to be fully created and ready for management. 

However, if it has been working before an stopped working, you may need to uncheck the Only allow messages from people inside my organization, save the settings, refresh the browser. Go back to the DG management again, check the Only allow messages from people inside my organization >> save the settings >> refresh the browser again and try reproducing the issue. 

Check if the emails does delivers to specific users, we need to check if these users has not added the external users as safe senders in their outlook.

In conclusion, if all the above has been tested and the issue is not resolved, we may be considering exporting members of the distribution group, deleting the distribution group and recreating a new one. If that still does not assist, then we would be looking at creating a Transport rule to block external emails from sending emails to the Distribution Group.