Had a request to setup a new domain for some new users. The intention was to allow these users to send from this domain to external users.
We operated a hybrid setup with Office365, and adding additional domains to this setup is pretty straight forward and there are tutorials on how to do this.. Pretty much add the domains to both side (exchange on premise, Exchange online and then run the Hybrid Config Wizard (HCW)).
An issue arises though when a user tries to send from the new domain email address. Whilst it will let them send it, when the email routes through exchange it will pick up the primary email address.. I thought I could force the reply to address to be the new email, but that didnot work, and the email arrives at the recipient and lists the primary email address..
So how to get round this...
It appears the only way around this is to setup the account to allow it to "send as" another email.. So there are two options shared mailbox, delegation group.
I chose the delegation group as I didn't want to create another mailbox. So I removed the alias from the user. Setup a delegation group and assigned the new email to that delegation group. Then gave "send as" permissions to the existing user to the new delegation group.
These permissions need to be setup both sides..
Exchange (on-premise)
Run AAD Connect (DirSync)
Exchange Online
