The call to ‘https://mail.domain.com/EWS/mrsproxy.svc’ failed because no service was listening on the specified endpoint.

Scenario: When migrating (onboarding) to Exchange Online, you may receive a error message similar to this:

The call to ‘https://servername/EWS/mrsproxy.svc’ failed because no service was listening on the specified endpoint. Error details: There was no endpoint listening at https://servername/EWS/mrsproxy.svc that could accept the message. This is often caused by an incorrect address or SOAP action. See InnerException, if present, for more details. –> The remote server returned an error: (404) Not Found.

Solution: Verify the ExchangeGuid is the same value:

Exchange OnPrem: Get-Mailbox <alias> | Select ExchangeGUID

Exchange Online: Get-mailuser <alias> | Select Exchange GUID

Fix – Exchange Online: Set-mailuser <alias> -exchangeguid <GUID of ExOnPrem user>