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External B2B collaboration users are prompted to login with #EXT# accounts when opening shared documents

Cory Spence 0 Reputation points
2025-10-23T20:27:18.7033333+00:00

Issue Summary

When external users—already provisioned as External Members in our tenant through an MTO sync—attempt to access shared documents from another Microsoft 365 tenant (that is part of the MTO), they are prompted to sign in using a #EXT# account. This identity is not usable for direct login, and users must manually select “Sign in with another account” and re-enter their original credentials to proceed.

This behavior is confusing and inconsistent with the expected seamless experience for pre-established guest identities.

🧭 Expected Behavior

We expect that pre-existing guest users:

  • Are automatically redirected to their home tenant for authentication.
  • Are prompted to sign in using their original identity (e.g., ******@externaldomain.com), not the #EXT# alias.
  • Experience a frictionless login flow without needing to manually override the default prompt.

❌ Actual Behavior

  • Users are prompted to sign in with the #EXT# account.
  • Authentication fails because the #EXT# account is not directly usable.
  • Users must manually override the prompt and re-authenticate.
  • If the user closes the window and reopens the link, the prompt disappears—suggesting session caching is involved.

📌 Impact

✅ Suggested Improvements

  1. Clarify documentation to explain this behavior for pre-existing guest users.
  2. Improve login flow to default to the user’s original identity, not the #EXT# alias.
  3. Provide configuration options to suppress the #EXT# prompt or streamline context switching.
  4. Offer admin controls to pre-map guest identities to home tenant credentials.

We appreciate your attention to this matter and welcome any guidance or roadmap updates that may address this issue.

Microsoft 365 and Office | SharePoint | For business | Windows
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  1. Rin-L 20,855 Reputation points Microsoft External Staff Moderator
    2025-10-24T09:03:28.56+00:00

    Hi @Cory Spence, 

    Thank you so much for posting your question in the Microsoft Q&A forum, and especially for taking the time to share such a clear, detailed summary of the issue you're encountering. Your insights into the user experience and the inconsistencies in the B2B collaboration flow are incredibly valuable, not just for the product team but also for others in the community who may be facing similar challenges. 

    You're absolutely right. This behavior can lead to a confusing and fragmented experience for external users, especially when they’ve already been provisioned and expect a seamless login flow. If the system could intelligently recognize and redirect users to authenticate using their original identity instead of defaulting to the #EXT# alias, it would significantly reduce friction and improve usability across tenants. Such an enhancement would not only streamline collaboration but also reduce support overhead and improve trust in cross-tenant sharing scenarios. 

    Personally, I completely agree with your perspective, and I believe many other users in the community would feel the same. However, as a forum moderator, I hope you can understand that I don’t have the ability to modify or escalate product issues directly. And because the community here is primarily focused on technical discussions/troubleshooting, I strongly recommend submitting your feedback via the Ideas · Community, where it will be routed directly to the product team for review and consideration in future roadmap updates. 

    Once you've posted your feedback, feel free to share the link here in the comments. I’ll be happy to vote for it to help bring more visibility.  

    If my response helped guide you toward the right place to raise this feedback officially, please consider marking it as the “Accepted Answer.”

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    This will pin the post to the top and make it easier for others in the community who may be experiencing the same issue to find your feedback and support it. Your voice matters, and collective feedback is a powerful way to drive improvements.

    Thanks again for being an engaged and thoughtful member of the community! 

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