Using classic Outlook for Windows in business environments
This points to either a domain/DNS problem in Microsoft 365 or an account sign-in mismatch after the custom domain was added.
- In the Microsoft 365 admin center, go to Setup > Domains.
- Check the Status column for the domain.
- If there is a warning or error, select the three dots (more actions) and choose Check health.
- Review the pane that opens. It shows which DNS records are wrong and what the records should be.
If the issue is specifically that Microsoft says the domain does not exist, the documented checks are:
- Verify the TXT verification record value is exact, including the MS= portion.
- Make sure the DNS change was actually saved at the DNS host.
- Allow time for DNS updates to propagate, which can take a few minutes and occasionally a few hours.
If Outlook is failing to set up the mailbox even though the domain exists, one known cause is Autodiscover being answered incorrectly by the domain’s website host. In that case, the web hosting provider needs to make sure the web server does not respond to Autodiscover requests for the root domain.
If the mailbox can still be opened only by using a different Outlook address and only very old mail appears, also check whether the Microsoft 365 sign-in name changed after the custom domain was added. A documented pattern is that after a new tenant adds and verifies a custom domain, the admin sign-in can change from the original .onmicrosoft.com address to the new custom-domain address, and the old sign-in can appear as if the account no longer exists. Try signing in in an InPrivate/Incognito window with the newer custom-domain username and the same password.
If Outlook desktop still does not connect after DNS is confirmed, use Microsoft 365 domain health first, then Outlook troubleshooting.