Emails stopped working in new outlook IMAP

Question

Hello,

I am experiencing an issue with IMAP accounts in the New Outlook. Some older accounts that have been working for years in Classic Outlook (desktop) have suddenly stopped working in the New Outlook, returning an “Unknown” error code when attempting to connect.

Important details:

• These accounts continue to work perfectly in Classic Outlook and webmail.

The SSL certificates are valid, and TLS 1.2+ is supported.

Newly created accounts on the same server work fine in the New Outlook.

The problem only affects older accounts; nothing has changed in their passwords or server configuration.

It seems that the New Outlook may have stricter requirements for account compatibility, which affect older accounts, even though they remain fully functional in Classic Outlook and webmail.

I would like to understand:

Why do older IMAP accounts fail in the New Outlook with an “Unknown” error, while new accounts work fine?

Is there any known workaround or configuration that can make older accounts compatible with the New Outlook?

Thank you for any guidance or advice.Hello,

I am experiencing an issue with IMAP accounts in the New Outlook. Some older accounts that have been working for years in Classic Outlook (desktop) have suddenly stopped working in the New Outlook, returning an “Unknown” error code when attempting to connect.

Important details:

These accounts continue to work perfectly in Classic Outlook and webmail.

The SSL certificates are valid, and TLS 1.2+ is supported.

Newly created accounts on the same server work fine in the New Outlook.

The problem only affects older accounts; nothing has changed in their passwords or server configuration.

It seems that the New Outlook may have stricter requirements for account compatibility, which affect older accounts, even though they remain fully functional in Classic Outlook and webmail.

I would like to understand:

Why do older IMAP accounts fail in the New Outlook with an “Unknown” error, while new accounts work fine?

Is there any known workaround or configuration that can make older accounts compatible with the New Outlook?

Thank you for any guidance or advice.

Answer 1

If the Uninstall, as advised by Jade Ng, does not help. You may need to:

1 -- Remove the Account from all devices

• View -> View settings -> Account info > Accounts > Your accounts
• Pick an Account and [Manage]
• [Remove]
• (o) Remove from all devices

2 -- Reset your New Outlook for windows Profile (the data in your Profile is not removed when you Uninstall).

See the answer by "Hornblower409 Dec 11, 2025, 12:11 AM" in
https://learn.microsoft.com/en-us/answers/questions/5655980/outlook-starting-error-repair-reinstall-reset-does

Answer 2

Dear @Bruno,

Good day. Thank you for posting your question in the Microsoft Q&A forum!

I sincerely apologize for the inconvenience this has caused and understand how disruptive it can be when email access is unexpectedly impacted.

Based on our review, this behavior is consistent with a known limitation in the New Outlook. While your IMAP accounts continue to function correctly in Classic Outlook and webmail, the New Outlook uses a different, cloud‑based connection and authentication framework. As part of this design, it applies stricter compatibility and validation requirements when establishing IMAP connections. In some cases, older IMAP accounts that were created years ago may fail this validation and return a generic “Unknown error,” even though the server configuration, credentials, SSL certificates, and TLS versions remain fully valid.

At this time, there is no supported configuration change or workaround that can force the New Outlook to accept these older IMAP accounts. As a result, we recommend one of the following options:

• Continue using Classic Outlook for these affected IMAP accounts, as it relies on the legacy Outlook IMAP stack and remains compatible.
• Use webmail or a third‑party email client that supports your current IMAP configuration.
• If feasible, create a new mailbox/account on the same server and migrate the existing mail data, as newly created accounts generally provision successfully in the New Outlook.

For reference:

Supported accounts in new Outlook for Windows

Getting started with the new Outlook for Windows

Feature comparison between new Outlook and classic Outlook

I understand this may not be the outcome you were hoping for, and I truly appreciate your patience and understanding. Please rest assured that your feedback is valuable, and I continue to monitor updates from the product team regarding IMAP support in the New Outlook.

Your feedback has been duly noted and is highly valued, as it provides with essential insights to continue improving and refining the products and services in the future. In this case, I sincerely recommend you go to Feedback community to vote this similar feedback: (find out similar feedback, if not provide the feedback forum link Ideas · Community). Our product team is in charge of the site, and they constantly check customer reviews and feedback. The higher votes, the more attention the related team will pay on. And a lot of the features are developed and improved based on customer feedback. 

Please note that the new Outlook is still in active development, with more features and enhancements in the works. Once again, I apologize for the inconvenience this has caused. I appreciate your patience and understanding and thank you for your time and cooperation.

If you have any questions or would like assistance with one of the recommended options, please feel free to let me know. I will be more than happy to help.

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