Outlook.com IMAP at outlook.office365.com:993 returns “NO User is authenticated but not connected” when Thunderbird uses OAuth2.

GREG 0 Reputation points
2026-10-15T05:18:43.61+00:00

Hello,

I am stuck with:

Outlook.com IMAP at outlook.office365.com:993 returns “NO User is authenticated but not connected” when Thunderbird uses OAuth2. Microsoft/YubiKey sign-in succeeds. IMAP is enabled, the username matches the primary alias, and the same failure occurs in a fresh Thunderbird profile. Re-saving IMAP access and testing without a VPN did not help. Please investigate IMAP mailbox access/connectivity.

See when and where you've used your account Does not show any IMap activity, just my "``Session activity:`` ``Successful sign-in".

When I try to log a support ticket, it asks for my account: https://outlook.live.com/mail/help/?prompt=select_account then outlook.live.com is blocked

outlook.live.com refused to connect.

ERR_BLOCKED_BY_RESPONSE

Please help. Thanks

Greg.

Outlook | Web | Outlook.com | Settings
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  1. Francisco Montilla 31,530 Reputation points Independent Advisor
    2026-10-15T08:43:38.96+00:00

    Hi Gregory,

    Just a quick heads up first. This is a public community forum, so everyone here is a fellow user and nobody has backend access to inspect or toggle mailbox routing on Microsoft's servers.

    That specific IMAP error means the OAuth2 token handshake succeeded at the identity layer, but the Exchange IMAP gateway failed to bind that session to an active mailbox store. Before looking into specific fixes, I need to check two things to diagnose where the route is breaking.

    • Can you log into your actual inbox normally in a regular browser window to send and receive emails, or does the webmail interface itself fail to load?
    • What domain extension is your primary alias using? Is it an official @outlook.com or @hotmail.com address, or is it a third-party address or custom domain?

    Please let me know :)

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  2. GREG 0 Reputation points
    2026-10-15T05:48:28.3333333+00:00

    I tried to connect to support page from Chrome (previouly brave), no ad block, still :

    https://outlook.live.com/mail/help/?prompt=select_account then outlook.live.com is blocked

    outlook.live.com refused to connect.

    ERR_BLOCKED_BY_RESPONSE

    So I am stuck all the way.

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  3. AI answer

    2026-10-15T05:19:00.1066667+00:00

    For Outlook.com with Thunderbird, the supported path is to use OAuth2, and Thunderbird should be configured with these settings:

    1. In Thunderbird, open ≡ > Account Settings.
    2. Under Server Settings for the Outlook.com account, set Authentication method to OAuth2.
    3. Under Outgoing Server (SMTP), select the Microsoft account, choose Edit, and set Authentication method to OAuth2.
    4. If the account was previously added with older settings, remove it and re-add it so Thunderbird detects the proper OAuth settings automatically.

    Also verify Outlook.com IMAP access is enabled:

    1. In Outlook.com, go to Settings > Mail > Forwarding and IMAP.
    2. Under POP and IMAP, turn on Let devices and apps use IMAP.
    3. Select Save.

    Because the account is reaching Microsoft sign-in successfully but IMAP still returns “User is authenticated but not connected”, the documented Outlook.com workaround for IMAP connection problems is to authorize the IMAP sign-in from the Microsoft account activity page:

    1. Go to account.live.com/activity.
    2. Find the most recent Session Type event matching the time of the IMAP failure.
    3. Expand it and select This was me.
    4. Try the IMAP connection again in Thunderbird.

    If no IMAP activity appears there and the same error continues even in a fresh Thunderbird profile, the remaining guidance in the available material is to remove and re-add the account in Thunderbird, ensure Thunderbird is current, and use Thunderbird community support for the client-specific localhost/OAuth callback behavior.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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