Please check the result running below command on your Exchange online server side. Test-MigrationServerAvailability is used to test the availability of the target server in preparation to perform cross-forest mailbox moves, include migration of mailbox data from an IMAP server to Exchange Online mailboxes.
$MRSEndpoints = (Get-MigrationEndpoint).Identity
Foreach ($MEP in $MRSEndpoints) {Test-MigrationServerAvailability -Endpoint $MEP}
To Create endpoint-
$Credentials = Get-Credential
New-MigrationEndpoint -Name Endpoint -Gmail -RemoteServer MRSServer.contoso.com -Credentials $Credentials
And if you are not familiar with how to Connect to Exchange Online PowerShell, please refer to the steps introduced in this official document.
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