Shared mailbox address not staying in "From:" dropdown in Outlook

Question

We have two domains, let's call them contoso.com and fabrikam.net. Both are set up in M365 admin center and can be used in Exchange Online. In this scenario we have the users' mailboxes (@contoso.com) and a shared mailbox (@fabrikam.net). When adding the users as members of the shared mailbox they appear in Outlook and can view received mails and answer to them normally. When sending new mail from the shared mailbox's address (******@fabrikam.net) users have to click on the "From:" button inside the message window. It only works if they search and select the shared mailbox from the Global Address List. If the instead enter the address manually, Outlook will send the message as "On behalf of". That is a little strange but would not be a problem. But the shared mailbox only gets added to the "From:" dropdown when entered manually.

When selected from the GAL it does not get added. Why are the addresses not added to the dropdown if selected from the GAL? Why are mails sent "On behalf" instead of "Send as" when entering the address manually instead of picking it from the GAL?

Answer 1

That is the expected behavior. If you want the shared mailbox address permanently listed under the From dropdown, you have to remove automapping and add the shared mailbox as additional account instead (File > Add account > enter shared mailbox address > enter delegate credentials when prompted). This has the added benefit of exposing all additional features as well, so you can work with rules, categories, OOO and so on, just like with any "regular" mailbox.