Ajay Joshi's Blog

Guest users can't sign-in on Office.com

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Ajay Joshi
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We created Guest accounts in Microsoft 365 and converted them to Members and changed their UPNs to the custom domain removing the #EXT# portion. Now, their UPNs are exactly like their personal email just that the domain is replaced with the tenant's custom domain. So, technically the Guest users are now Mail users. To understand this, consider the following example:-

External email:

Azure AD Guest UPN: joshixxxxxx_gmail.com#EXT#@xxxxxx.onmicrosoft.com

New UPN after conversion to Member:

Also enabled SSPR through which these Mail users can reset their passwords for signing into Office 365. However, some users were getting the following error:-

This happens when the Guest account is created through an external non-Microsoft directory i.e. Gmail or Yahoo and not with an Azure AD or a Microsoft account. Hence, you can only use your personal email address for sign-in and not the Azure AD UPN. You will receive a passcode on that email for authentication, a password will not be set or required for sign-in here. Ref article - One-time passcode authentication for B2B guest users - Microsoft Entra | Microsoft Learn

You will not be able to sign in at Office.com directly as well, please follow these steps instead:-

  1. Go to Office.com and click on Sign-in Options(at the bottom).

  2. Then choose "Sign in to an organization".

  3. Then type the tenant's custom or initial domain.

  4. After clicking next, the usual sign-in page will appear, now you have to use your personal email address as it is. Please remember you can't use your Azure AD/M365 UPNs here, although the sign-in looks exactly the same as Office.com

You can also skip this whole process, just use the link provided in your Invite email, above the "Accept Invite" button.

To simplify this link even more, you can replace the complex Tenant ID with the custom or initial domain like voltaze.com:-

https://myapplications.microsoft.com/?tenantid=voltaze.com

You will not face this issue if your personal email was already registered as a Microsoft account before accepting the Guest invite.

Microsoft365AzureADAzure AD Guest user