Forum Discussion
Exchange Online Shared Mailboxes deleted after deleting disabled users from local AD
Here is the support article from Microsoft outlining what you were trying to do (with the warning not to delete the user) as well as the steps to recover if you did delete the user - https://support.office.com/en-us/article/Convert-a-user-mailbox-to-a-shared-mailbox-2e122487-e1f5-4f26-ba41-5689249d93ba
I'm also not aware of it being a license violation based on everything I've seen on the topic. I know all the users that access the shared mailbox must have a license, but not aware of any violations of using a shared mailbox to preserve a mailbox, especially if you need to continue receiving email to said mailbox. Here is another good article on the topic as well https://practical365.com/exchange-online/shared-mailboxes-vs-inactive-mailboxes-departed-users/. Still doing some looking into the licensing issue, so I'll update the thread as well if I can find any more details around it.
Hopefully this clears it up:
1. An on-prem AD account with an Exchange Recipient Type Status of User Mailbox
2. An on-prem AD account with an Exchange Recipient Type Status of a Shared Mailbox
3. A Cloud Identity User Mailbox
4. A Cloud Identity Shared Mailbox
#3 and #4 do not require anchors because their 'source of authority' is Azure AD instead of on-premises AD.
So the reason why your Synced Accounts from on-prem AD require anchors is because their source of Authority is set to on-premises AD (regardless of whether it has Recipient Type status of User Mailbox or Shared Mailbox) .
Converting an Office 365 mailbox from user to shared does not change the source of authority from on-prem to Cloud.
So it makes perfect sense to me that your other 5 shared mailboxes that are not synced from on-premises AD don't require anchors, it is because they were born in the cloud to start out with.
Great explanation, thank you.