After migrating a customer to Microsoft 365 about six weeks ago, we've landed in a frustrating situation. Even though the custom domain has been verified in the M365 Admin Center and all the necessary DNS records (like MX, SPF, and autodiscover) are correctly set up and confirmed via tools like MXToolbox, we're still facing some significant issues.
- Login only works using the `.onmicrosoft.com` email address, and attempts to sign in with `[email protected]` are failing.
- When connecting with `[email protected]`, instead of redirecting to Exchange Online with OAUTH, it unexpectedly falls back to IMAP.
- Autodiscover tests are also not resolving correctly for the custom domain, suggesting that Exchange Online doesn't recognize the domain for mailbox login.
It seems like while the domain is verified at the tenant level, Exchange Online hasn't fully registered or activated it from its end, leading us to suspect it's only "half-verified." Is there a way to force Exchange Online to register the domain fully for login and authentication purposes? Any insights would be hugely appreciated!
5 Answers
Just an idea—were these users migrated from an on-premises Active Directory? Because I've witnessed cases where synced users sometimes have login issues unless properly configured for the cloud.
These are new users, and I'm still hitting a snag with Outlook trying to set up via IMAP. (shares a screenshot)
Have you checked if the primary email address was updated without adjusting the User Principal Name (UPN)? Not changing the UPN could lead to the behavior you're seeing, where it defaults to logging in with the `.onmicrosoft.com` address instead.
The login is now set to the onmicrosoft.com address because the `[email protected]` wasn't working, so it feels kind of stuck.
Try logging in from a device outside of your customer's network to rule out any local DNS issues. Sometimes, records can be manually added that cause conflicts.
I thought I'd checked all that already, but it seems there are still some problems I need to sort out.
Here's what happens when I try logging in: (sends an error screenshot)
Can you access Outlook Web App (OWA) with the custom domain? You might want to run a check using Microsoft's connectivity test tool to diagnose the issue further.
I can access OWA, but only with the onmicrosoft.com email, not the custom one.
Make sure the user accounts are configured correctly. Are their primary logon emails aligned with the custom domain? Have you tried setting up a fresh user object with the custom domain as the main identifier to see if it resolves the logging in issue?
No, this was an IMAP to Exchange Online migration, so that's not the case.