Back in 2017, we announced that site mailboxes in SharePoint Online were being deprecated, and that new organizations would no longer have access to the feature and existing organizations would no longer be able to create site mailboxes. Since that announcement, we have seen site mailbox usage drop consistently as customers move to alternative solutions. There are still a few customers who have site mailboxes with some activity, as well as customers who want to archive and delete site mailboxes that are no longer in use.
Before we retire (and ultimately delete) all site mailboxes in SharePoint Online, which will happen in April 2021. We’ll post another update as we get closer to April 2021. In the meantime, you can back up the data in your existing site mailboxes using this instruction guide.