Failed mailbox migration to Office 365: mrsproxy.svc’failed because no service was listening on the specified endpoint. The remote server returned an error: (404) Not Found

I was doing a migration between Exchange 2013 and Office 365 in a Hybrid configuration when I recieved the above error message. Couldn’t quite figure out why until I stumbled accross a forum thread that pointed me in the right direction.

This is what you have to check out and remediate if you have this error:

The ExchangeGuids of on-premise users are different to the ExchangeGuids of the corresponding users in Office 365.

Update the online user’s ExchangeGuid to match the on-premise ExchangeGuid and start migration.

1. On the on-premise Exchange server:

Get-MailBox -Identity userID | Select ExchangeGuid

2. In an Office 365 PowerShell session:

Get-MailUser -Identity UserID | Select ExchangeGuid

If the results don’t match, copy the guid result from command 1 and then run the following command in the Office 365 PowerShell session:

Set-MailUser -Identity userID -ExchangeGuid “copied guid”

Start migration:

$Cred = Get-Credential

$s = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri -Credential $cred -Authentication Basic -AllowRedirection

Import-PSSession $s

$OnPremAdmin = Get-Credentials

New-MoveRequest -identity “UPN” -Remote -RemoteHostName “remote host ex OWA URL” -RemoteCredential $OnPremAdmin -TargetDeliveryDomain