We're migrating from 2013 to 2019 and so far everything is going well. I've moved a good set of our user mailboxes with no issues and connectivity, mail flow, etc.
What I keep running into is these hidden arbitration mailboxes that I had no clue existed that need to be migrated and this got me thinking about what else I need to migrate before decommissioning our 2013 instance.
Currently, I'm aware of migrating: User, Room Resource, Shared Mailboxes. Public Folders.
Is there a comprehensive list of what needs to migrated and what doesn't? Is there command I should be running right before our 2013 instance is uninstalled and decommissioned to catch these things and migrate them?
I've been scouring the internet to try to find anything but most articles just simply say move all your mailboxes and public folders, turn off 2013 for a week. Then turn it back on if there are no issues and run the uninstaller to remove 2013 from the domain appropriately.
You should move arbitration mailboxes first, before you move user mailboxes.
Here is a similar case you could refer to: When to Move arbitration mailboxes ?
Also, do you use Exchange Deployment Assistant to help with mailbox migration? If not, you can refer to it: On-Premise Exchange Deployment Guide
It has been about 4 years since I went from 2010 -> 2016 -> 2019, but I don't THINK the arbitration mailboxes are required to be migrated. That said if you do need to move them Exchange 2013 has 5 arbitration mailboxes and there's even more in 2019. You can use
Get-Mailbox -Arbitrationto see all of them.From what I recall of my migration I hit the same mailboxes you did and didn't have any issues. I also used this guide which was very helpful, https://practical365.com/migrating-exchange-server-2016/.