Shared mailbox should have an account associated with it. Just reset the password and give it license and you should be good to go.

Of course, that was far too simple! I had gone down a path of using Azure OAuth Authenticaiton etc.
Didn’t even need to have a licence, just had to have that password sorted!

For Microsoft Shared Mailboxes to work as Exchange system emails in Acumatica/MYOB, these must first begin as a Regular mailbox. They do not need to be licensed, they can be shared, but only after they have been logged into once as a regular mailbox. The act of logging into the account requests the user to provide a phone number and receive a text, and without that it will not work - it seems that this step is needed  to initialize the account at Microsoft.

After that, the account can be switched to a shared mailbox and the license removed.

In the future Microsoft may require a license on shared mailboxes, but that is easy enough to do, but for now it is not required. 

How do we go about using an existing sharedmailbox that was in production prior to Acumatica implementation? All users using the sharedmailbox are licensed, and my understanding of licensing is that once I assign (even an email only license to the account) it is no longer a sharedmailbox and defeats the purpose. 

I’ve gone through the process of “resetting” a password on a sharedmailbox and gone through authentication with M365, only to result in a “mailbox not found” result directly at outlook.office365.com.

I know I'm missing something simple here, but we obviously don’t want to remove access or interrupt processes with existing sharedmailbox, but I'm failing to understand the steps to take to connect these mailboxes to Acumatica.