Consider a large organisation, where deleted mailboxes are kept for many years.

Consider a new user with the same name as an offboarded user, or a user getting rehired, but policy stating that the deleted mailbox should not be restored. The user should start with a clean mailbox.

The inactive old mailbox still has the standardized primary smtp / alias etc and will not allow you to set these on the new user, causing a conflict.

Of course this should be handled during the offboarding process, where perhaps the email address of the user could be appended with _old123 before being soft-deleted.

Since that wasn’t the case here, I had to write a quick script to retroactively add a random string to all such attributes for all inactive mailboxes, hope it helps someone else with the same legacy 🙂

connect-exchangeonline
$inactiveMailboxes = Get-Mailbox -InactiveMailboxOnly -ResultSize unlimited

foreach($inactiveMailbox in $inactiveMailboxesNovib){
    $rand = Get-Random -Maximum 999
    $primary = $inactiveMailbox.PrimarySmtpAddress
    $newMailbox = New-Mailbox -InactiveMailbox $inactiveMailbox.DistinguishedName -name $inactiveMailbox.Name -FirstName $inactiveMailbox.DisplayName.Split(" ")[0] -LastName $inactiveMailbox.DisplayName.Split(" ")[1] -DisplayName $inactiveMailbox.DisplayName -MicrosoftOnlineServicesID $inactiveMailbox.PrimarySmtpAddress.Replace("@","_old$rand@") -Password (ConvertTo-SecureString -String 'W1pos3wsd03?!' -AsPlainText -Force) -ResetPasswordOnNextLogon $true    

    $newMailbox | Remove-Mailbox -Force -Confirm:$False
}