Problems signing into Microsoft 365

Sam Coleman 20 Reputation points
2026-09-08T22:19:55.69+00:00

Hi there,

I recently purchased a custom domain from 123 reg, I then went on to purchase a basic business package from the Microsoft 365 website. I went through all of the process of adding the custom domain into my Microsoft 365 account, I have now completely lost my email address. The domain Microsoft created for me says that my username may be incorrect when I try to log in, I have spent 2 full days trying to sort this out and Microsoft have been unable to help me. Can someone please help me?

Microsoft 365 and Office | Subscription, account, billing | For business | Other
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Answer accepted by question author
Kai-L 18,970 Reputation points Microsoft External Staff Moderator
2026-09-08T22:36:28.8633333+00:00

Dear Sam,

As a forum moderator, I genuinely wish I could directly access your account or delve into the backend systems to diagnose and fix this for you. Unfortunately, my role here is limited to providing general guidance and solutions that can be applied by users. One thing worth knowing straight away is that adding a custom domain does not disable the original .onmicrosoft.com account created when you signed up. That account remains valid and is normally the way back into the admin center, so if you are only trying the new custom domain address, that alone could explain the sign in failure. A common cause is that the custom domain was added and verified, but no user was ever created on it or no licence was assigned, so there is no mailbox behind the address yet. To narrow it down, could you confirm a few things?

  1. What is the exact error message when you try to sign in?
  2. Can you sign in at admin.microsoft.com using the original .onmicrosoft.com account?
  3. Are you trying to reach Outlook email, the admin center, or both?
  4. Was the domain previously hosting email elsewhere, or is Microsoft 365 the first mail service on it?

If you no longer have the .onmicrosoft.com username, check your mailbox for the welcome message Microsoft sent when the subscription was created, as it usually contains it.

Once I have a clearer picture, I will do my best to provide you a clear path forward.

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