Hello @Chris Parke，
Welcome to our forum!
The error message you provided indicates that the Mailbox Replication Service was unable to connect to the remote server using the credentials provided. This issue seems to be related to the credentials that you used for migration endpoint. It may be caused by the wrong password for the migration endpoint on On-Premises end.
In order to resolve the issue, it is recommended that you try the following steps:
1.Open Office 365 Exchange Portal – Recipients- Migration – Click on Migration endpoints. Update the Migration Endpoint Password – which has access to On-Prem Mailboxes. Then rerun the migration.
2.Use the Test-MigrationServerAvailability command to see what messages you get, and then troubleshoot according to the link on the Exchange team blog about solving this issue. You may could refer to "Option 3: Test-MigrationServerAvailability fails with 401 Unauthorized, Access Denied, or Invalid Credentials section.
3.Please verify that the credentials you are using are correct and have sufficient permissions to perform the migration.
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