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Yes, every mailbox is linked to a user object, and if you are syncing from on-premises you'll have that dependency too. Moving the users to a different OU will cause to be deleted in AD, although you can recover the user from the Recycle bin after the deletion. If you want to preserve mailbox data, best look at the Inactive mailboxes functionality: <a href="https://docs.microsoft.com/en-us/microsoft-365/compliance/inactive-mailboxes-in-office-365?view=o365-worldwide" rel="ugc">https://docs.microsoft.com/en-us/microsoft-365/compliance/inactive-mailboxes-in-office-365?view=o365-worldwide</a><br />
If for some reason you still need to use shared mailboxes, either take the risk of deleting/recovering the object, or disable dirsync, clear the immutableID attribute and re-enable dirsync.
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Smart Sand

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I appreciate that information above, but I failed to mention something in my previous message. &nbsp;I also need to make sure that the mailboxes are available "natively" in outlook, which is why the .PST wouldn't work. &nbsp;I had proposed just loading the PST into the users mailbox so it would be in outlook but that was not in scope of the policy. &nbsp;Would the inactive mailboxes still be able to be accessed, or would they need to be converted back to an active mailbox to navigate through the data?
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The content of Inactive mailboxes can be accessed, but not directly.<br />
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It must be searched, then exported, then proposed to users.<br />
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Shared mailboxes are probably a better solution. They don't need a license, but emails are still working. So, the shared mailboxes &nbsp;can still receive messages and be modified.
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<p>As part of our process for handling mailboxes for <span style=" text-align: left;">terminated users</span>, we are converting the users' mailboxes to a shared mailbox and assigning the appropriate member(s) access to the shared mailbox. &nbsp;As it stands, we have a lot of stale users in our on-prem AD infrastructure, as the terminated users are just disabled and moved to a different OU. &nbsp;I am wondering if after the mailboxes are converted to a shared &nbsp;mailbox, does the AD account need to remain in order to retain all the previous emails in the shared mailbox. &nbsp;If it's not as simple as just deleting the old users after converting their mailbox to a shared mailbox, is there a different process to keep the AD clean of disabled users, while retaining their mail? &nbsp;Downloading and retaining the .PST files is not a viable solution for our company, so I've already ruled that out.</p>
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As part of our process for handling mailboxes for
terminated users
, we are converting the users' mailboxes to a shared mailbox and assigning the appropriate member(s) access to the shared mailbox.  As it stands, we have a lot of stale users in our on-prem AD infrastructure, as the terminated users are just disabled and moved to a different OU.  I am wondering if after the mailboxes are converted to a shared  mailbox, does the AD account need to remain in order to retain all the previous emails in the shared mailbox.  If it's not as simple as just deleting the old users after converting their mailbox to a shared mailbox, is there a different process to keep the AD clean of disabled users, while retaining their mail?  Downloading and retaining the .PST files is not a viable solution for our company, so I've already ruled that out.

Does a user converted to a shared mailbox still require an AD account.

Office 365 is a group of software plus services subscriptions that provides productivity software and related services to its subscribers. Office 365 allows the use of Microsoft Office apps on Windows and OS X, provides storage space on Microsoft's cloud storage service OneDrive, and grants 60 Skype minutes per month. Office 365 includes e-mail and social networking services through hosted versions of Exchange Server, Skype for Business Server, SharePoint and Office Online, integration with Yammer, as well as access to the Office software. All of Office 365's components can be managed and configured through an online portal; users can be added manually, imported from a CSV file, or Office 365 can be set up for single sign-on with a local Active Directory using Active Directory Federation Services.