I have been asked to migrate email for 8 users from an on-prem Windows SBS 2011 Server (Exchange 2010) to M365. Because of the small number of mailboxes to migrate I chose a cutover migration.
The SBS server has a trusted SSL cert. Mailbox access via OWA is working, and users can access their email via smartphone.The M365 tenant already exists and the domain is verified.
I am stuck creating the migration endpoint in Exchange Online, getting the error
Failed to create migration endpoint.
Error: The last connection attempt happened too recently. Please wait until '06/23/2026 14:19:01' before trying to connect to an endpoint.
Waiting and retrying results in the same error message.
I used Remote Connectivity Analyzer (Test Outlook connectivity) to check if the on-prem server is accessible; it was successful with warnings and the only warning was:
Certificate trust is being validated.
The certificate is trusted and all certificates are present in the chain.
Test Steps
The Microsoft Connectivity Analyzer is attempting to build certificate chains for certificate CN=remote.mydomain.com.
One or more certificate chains were constructed successfully.
Additional Details
Analyzing the certificate chains for compatibility problems with versions of Windows.
The test passed with some warnings encountered. Please expand the additional details.
Additional Details
The Microsoft Connectivity Analyzer can only validate the certificate chain using the Root Certificate Update functionality from Windows Update. Your certificate may not be trusted on Windows if the "Update Root Certificates" feature isn't enabled.
I have also tried Test-MigrationServerAvailability in powershell but this fails with a 404:
> Test-MigrationServerAvailability -Credentials $cred -ExchangeServer SERVER.mydomain.local -RPCProxyServer remote.mydomain.com -ExchangeOutlookAnywhere -EmailAddress [email protected] -Authentication Basic
Test-MigrationServerAvailability: Server returned HttpStatusCode.NotFound failure. [HttpStatusCode=404 Not Found] [LID=89372] ###### REQUEST
[2026-06-23T13:56:10.1748432Z] [ResolvedIPs: 157.125.x.x] ###### POST /mapi/nspi?useMailboxOfAuthenticatedUser=true
HTTP/1.1 client-request-id: a1c0545e-ec85-4c68-9924-ceb9cc9f7f7c X-RequestType: Bind X-RequestId:
9f235dfc-d0de-435d-9e4e-a87c0924a3dd:1 X-ClientApplication: MapiHttpClient/15.21.139.8 X-ClientInfo:
2d4ccc06-6a7d-43df-b0d3-dd02050ef43a:1 User-Agent: MapiHttpClient traceparent:
00-f9ce8343a1ae1b8fd1a31b17da71ac32-98a9ebf4d595b399-00 Content-Type: text/html Content-Length: 1245 --- REQUEST BODY
[+0.001] --- ..[BODY SIZE: 45] --- REQUEST SENT [+0.001] --- ###### RESPONSE [+0.106] ###### HTTP/1.1 404 Not Found
Server: Microsoft-IIS/7.5 X-Powered-By: ASP.NET Date: Tue, 23 Jun 2026 13:56:10 GMT Content-Type: text/html
Content-Length: 1245 --- RESPONSE DONE [+0.106] --- ###### EXCEPTION THROWN [+0.106] ######
Can anyone suggest further troubleshooting steps?