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Thanks for the feedback.
See this thread for support when primary / only Office365 admin is deleted:
Office365 support can reconnect a mailbox if the entire mailbox has been accidently deleted, if the deletion has taken place within 30 days of the request.But there is no mail recovery procedure in Office365.
I didn't have any data in the mailbox, so recovering the mail is not an issue. However, it seems like you should be prevented from deleting the last/only administrator user in the tenant.