Hi RoccoT,
To add a proxy address to a mailbox, please refer to the following steps:
1. In the ECP (Exchange Control Panel) page, under the “Mailboxes” section, select the mailbox which you want to add a proxy address to and click “Details”.
2. In the “Mailbox” window,” under the “E-Mail Options”, click “Add”.
3. Enter the proxy email address.
Here is a picture for your reference.
Moreover, if you cannot add the proxy address via the above steps, I’d like to confirm that if you have configured the DirSync.
In addition, please capture a screenshot of your “E-Mail Options” and post it in your update.
Thanks,
Allen Zhang