https://learn.microsoft.com/en-us/powershell/module/exchangepowershell/new-mailboxrestorerequest?view=exchange-ps
New-MailboxRestoreRequest (ExchangePowerShell) | Microsoft Learn
When mailboxes are moved from one database to another, Exchange doesn't fully delete the mailbox from the source database immediately upon completion of the...
newmicrosoftlearn
https://support.microsoft.com/en-au/topic/event-id-4999-and-nullreferenceexception-when-the-new-mailboxrestorerequest-and-new-mailboxexportrequest-cmdlets-fail-in-exchange-server-2016-092f665a-dc24-3a10-14c0-175ee94d26bf
Event ID 4999 and NullReferenceException when the New-MailboxRestoreRequest and...
Event ID 4999 and NullReferenceException when the New-MailboxRestoreRequest and New-MailboxExportRequest cmdlets fail in Exchange Server 2016.
event idthe newnullreferenceexception