We have some users that have multiple inboxes in Outlook. Usually, they will have their individual work email and a team vanity/shared inbox (i.e. firstname.lastname@example.org)
. With Salesforce for Outlook they could use the side panel across email inboxes, but they can't do the same with Lightning for Outlook.
From the Lightning for Outlook PM at salesforce:
"I've looked into the Office Add-In documentation (the framework that Lightning for Outlook is written in), and it appears that those special mailboxes are not in their support scope. That explains why your IT team was not able to enable it there. I've talked to my PM counterpart at Microsoft and they have not slatted that work to a specific release at this time."