I've got a client using Exchange 2010 on premise.
They want to be able to Send As from multiple addresses without setting up an Exchange account for each address.
They have their primary email address e.g.
I've set up mail enabled security groups for:
And set the permissions to 'Send As' in the Active Directory Admin Centre.
However they keep getting bouncebacks:
You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission.
Am I missing something?
I followed this guide initially: