I have a small DG firstname.lastname@example.org and a big DG email@example.com. I configured the Delivery management of firstname.lastname@example.org only with email@example.com. But the members of abc still able to send email to the group firstname.lastname@example.org
How can I restrict them ?
Can you re-read your original post as I don't think I understand the scenario. Better yet, share the corresponding DG properties:
Get-DistributionGroup DG | fl Accept*,Require*,Reject*