Use this command to list all the inactive mailboxes in your tenant. You will also have to use this command in order to restore an inactive mailbox to a new user account. You will need to run the command below to gather information on the inactive mailbox and then use the Recover an Inactive Mailbox in Office 365 (Exchange Online) post.
List Inactive Mailboxes
- Get-Mailbox -InactiveMailboxOnly | Format-List Name,DistinguishedName,ExchangeGuid,PrimarySmtpAddress