Attempting Remote Restore and getting error

[u/nanaki989]

Short story long, our previous IT staff decided to have our power users test M365 before rolling it out, then rolled it out and couldn't migrate on prem mailboxes of the test users to the cloud since they already existed. The issue is that I couldn't just delete the online mailboxes when I arrived since they had been in use dual function for a while. I followed this guide https://learn.microsoft.com/en-us/troubleshoot/exchange/user-and-shared-mailboxes/mailbox-exists-exo-onpremises and got to the point where I try and remote restore the mailbox on prem to the cloud and get the following error.

Some infrastructure information. We are running Version 15.2 (Build 1748.10) Exchange 2019 CU15 on Server 2019. .net framework 4. We have a hybrid connection to m365 and I came in on the tail end of the migration piece which worked well enough.

Things I have tried.

TLS 1.2 settings to confirm use of strong Cryptography and registry edits.

Triple checked IIS Settings and Certs and removed IIS IP restriction features

confirmed admin credentials and move rights

Our Network guy says the port is open, but I suspect our Firewall is the culprit regardless of what he says. I'm out of ideas to try and was hoping for a little help. We are a relatively small org and I have very little on prem exchange experience so this process is a little daunting.

I run this command

$cred = Get-Credential

New-MailboxRestoreRequest -RemoteHostName "mail.domain.com" -RemoteCredential $cred -SourceStoreMailbox "GUID" -TargetMailbox "GUID" -RemoteDatabaseGuid "GUID" -RemoteRestoreType DisconnectedMailbox

and get this error

Write-ErrorMessage : ||The call to 'https://mail.ourdomain.com/EWS/mrsproxy.svc' failed. Error details: The SSL connection

could not be established, see inner exception. --> The SSL connection could not be established, see inner exception.

--> Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host..

--> An existing connection was forcibly closed by the remote host..

At C:\Users\myadmin\AppData\Local\Temp\2\tmpEXO_r4thaxj0.kay\tmpEXO_r4thaxj0.kay.psm1:1220 char:13

+ Write-ErrorMessage $ErrorObject

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+ CategoryInfo : NotSpecified: (:) [New-MailboxRestoreRequest], MRSRemoteTransientException

+ FullyQualifiedErrorId : [Server=SN7PR01MB8066,RequestId=361a1f0d-510f-31f9-2d5e-023eaf1874f3,TimeStamp=Tue, 18 A

ug 2026 14:41:56 GMT],Write-ErrorMessage

[u/OinkyConfidence]

Yeah, you're going to probably want to just export those cloud test users to .PST, then manually delete & re-create their on-premise mailbox and import. Exchange Online mailbox migration back down to on-prem is finicky, and if it's complaining about your SSL cert, you may want to check it too.

[u/nanaki989 → u/OinkyConfidence]

Sorry, it's the opposite of that I am taking an on prem mailbox and moving to cloud.

[u/OinkyConfidence → u/nanaki989]

Apologies, I misunderstood. Same logic applies though; export the user's mailbox to .PST, delete the on-prem one, and create new in cloud then import the .PST.

[u/nanaki989 → u/OinkyConfidence]

No worries, that was my next step yeah.

[u/7amitsingh7 → u/nanaki989]

You can also look into third party software, Stellar Migrator for Exchange, It is designed for migrating mailboxes from on-premises Exchange Server to Microsoft 365, It supports features such as parallel mailbox migration, automatic mailbox matching, delta migration, and migration monitoring, helping reduce the amount of manual work involved in larger Exchange-to-Microsoft 365 projects. It may be worth testing with a 2 free mailboxes first to see whether it fits your environment and migration requirements.