In the new Outlook app for Windows, a specific IMAP account cannot be added - INVALIDCREDENTIALS INTERACTIONREQUIRED

Hi Philipp Harnisch,

Welcome to Microsoft Q&A forum. 

Since you’re using the new Outlook for Windows, please note it handles third‑party IMAP differently from classic Outlook and may require the “Sync to the Microsoft Cloud” connection flow for IMAP accounts. Because of that, resetting/reinstalling often won’t change the outcome if the incident is related to the auth/sync flow for this specific mailbox.

Could you share the details below so we can narrow down what’s happening?

• Do you see the “Sync to the Microsoft Cloud” prompt when adding the account, and does it fail before/after you continue?
• Does the mailbox password contain any non‑ASCII/Unicode characters?
• The IMAP server log/result at the time of the failed sign‑in, plus your new Outlook version/build.

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Thank you for your time and understanding.

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The error INVALIDCREDENTIALS INTERACTIONREQUIRED in new Outlook for Windows when adding a specific IMAP account, while the same account works in Classic Outlook and other clients, matches known behavior where new Outlook requires an additional authentication step (often via an app password or provider-side consent) that is not being completed or not being triggered correctly for that mailbox.

From the available information, this is most consistent with a provider-side authentication/consent issue for that one mailbox rather than a general IMAP configuration problem, because:

• Other IMAP accounts on the same server work in new Outlook.
• The same account works in Classic Outlook and other clients using the same IMAP/SMTP settings.
• New Outlook has specific logic around app passwords and third‑party accounts, and there are known issues that manifest as credential-related errors.

Actionable steps:

1. Check whether the provider requires an app password for this mailbox
   New Outlook may require an app password for some third‑party or IMAP accounts even when the basic IMAP settings are correct.
   • Follow the provider’s instructions to generate an app password for this mailbox (not the main account password).
   • Use that app password when adding the account in new Outlook instead of the normal password.
   • For reference, the documented behavior for new Outlook is that some IMAP/iCloud/third‑party accounts need an app password: see the “App password message” guidance in the add‑account article.
2. Confirm that this mailbox is not blocked or requiring extra consent on the provider side
   Because other mailboxes on the same server work, check the provider’s web portal for this specific mailbox:
   • Sign in to webmail for this mailbox and look for any security prompts, consent dialogs, or account‑protection notices that must be accepted.
   • Ensure IMAP access is fully enabled for this mailbox (if the provider exposes per‑mailbox IMAP/security toggles). Even if IMAP is enabled globally, some providers allow per‑account restrictions.
3. Re-attempt add with a clean auth state in new Outlook
   Even though Windows Credential Manager has no entries for this account, new Outlook maintains its own cloud‑based and app‑local auth state.
   • Remove any partially added instance of this account from new Outlook.
   • Restart new Outlook and add the account again, using the app password if one was created in step 1.
4. Check for known new Outlook issues with third‑party accounts
   There is a documented issue in new Outlook where users see an error like “You may need to make an app password” when adding or using some third‑party accounts; this has been fixed in current builds. Ensure:
   • New Outlook is fully updated to the latest version.
   • After updating, retry adding the account.
5. Compare with a working mailbox on the same server
   Since another mailbox on the same server works in new Outlook:
   • Compare all server‑side security settings between the working mailbox and the failing one (IMAP enablement, any 2FA/app‑password requirements, IP or client restrictions, etc.).
   • Align the failing mailbox’s settings with the working one, then retry adding it to new Outlook.

If, after confirming provider‑side settings and using an app password where required, the account still fails only in new Outlook with INVALIDCREDENTIALS INTERACTIONREQUIRED, this behavior is effectively a product limitation/bug in how new Outlook handles the specific auth flow for that mailbox. In that case, the practical workarounds are:

• Continue using Classic Outlook for this mailbox, where it works reliably.
• Use another mail client for this mailbox until a future new Outlook update changes how it handles this provider’s authentication for that specific account type.

References:

• Add an email account to Outlook for Windows
• Error, "You may need to make an app password," when you add an account or send/receive email from some third-party email accounts