UNRESOLVED REGRESSION: Pre-Existing IMAP Accounts Fail in New Outlook ("App Password" Error) while Fresh Test Accounts on Same Server Connect Instantly

Question

UNRESOLVED REGRESSION: Pre-Existing IMAP Accounts Fail in New Outlook ("App Password" Error) while Fresh Test Accounts on Same Server Connect Instantly

IO (Israel O.) 0

Hello,

Reference Thread (Unresolved Prior Report):

Existing IMAP accounts inaccessible via new Outlook app (Q&A #4704670)

Environment & Scope:

Application: New Outlook for Windows (Desktop Client)
OS / Platform: Windows 10 / Windows 11 PC Desktop
Server Protocol: Standard IMAP (Port 993, SSL/TLS) & SMTP (Port 465, SSL/TLS)
Hosting Platform: Shared Linux Hosting / Plesk (Dovecot / Courier IMAP)
Impact: High-volume operational impact affecting standard non-M365 IMAP email clients across our IT customer base.

Description of Issue:

As highlighted in thread #4704670 (which was never resolved in full), users attempting to configure or authenticate pre-existing IMAP email accounts on New Outlook for Windows on PC encounter an unrecoverable login loop with the false error prompt:

"Something Went Wrong. Try Again. You may need an App Password"

Our underlying Linux/Plesk IMAP servers do NOT use or issue App Passwords.

Webmail, mobile email apps (iOS/Android), Classic Outlook on PC, and Mozilla Thunderbird all connect and authenticate cleanly using the exact same login credentials. Only New Outlook for Windows on PC fails.

Reproducible Isolation Test (Sanitized Setup):

To demonstrate that this is an internal client/SASL token regression, both test mailboxes reside on the exact same domain and mail server node:

FAILING PRE-EXISTING LEGACY ACCOUNT:
- Account Identifier: user_legacy@[customer-domain].com
  - Webmail Behavior: Logging into Webmail succeeds instantly.
    - New Outlook Behavior: Permanently fails authentication in New Outlook for Windows on PC with the "App Password" prompt.
    1. WORKING NEWLY CREATED TEST ACCOUNT:
      - Account Identifier: test_fresh@[customer-domain].com
        
        New Outlook Behavior: Connects IMMEDIATELY in New Outlook for Windows without asking for an App Password or throwing an error.

Technical Breakdown & SASL Session Payloads:

Because a newly created mailbox on the exact same server connects instantly, the issue is not caused by password complexity, firewall rules, server IP blocks, or missing Autodiscover. Instead, New Outlook's internal credential manager fails to negotiate authentication on pre-existing account identifiers.

Captured Session Error Payloads:

{"Error":"INVALIDCREDENTIALS INTERACTIONREQUIRED"}
{"Error":"UNKNOWN undefined"}

Urgent Escalation Request:

As a Support Supervisor at an IT hosting company managing infrastructure for a large volume of affected business clients, we need Microsoft Outlook Engineering to address this bug without standard back-and-forth scripts.

Please reply directly to this thread or contact the registered email address on this Microsoft Q&A profile. I am prepared to provide live diagnostic logs, console outputs, and temporary test account access to assist engineering in deploying a fix.

Thank you,

IO

Hornblower409 15,330 Reputation points

2026-09-15T04:57:57.4733333+00:00

Are you saying that - You have an IMAP account that was added to New Outlook for Windows and was working OK. But you have changed the hosting service and Outlook keeps trying to connect to the old host?

If so, I think I'm working a similar problem on Reddit. Is this your issue?
[External Non-Microsoft Site] https://www.reddit.com/r/Outlook/comments/1wel696/new_outlook_changing_the_imapremoteserverinfo_is/

Have you brought in Microsoft Support to see if they can clear the cached settings on the cloud mail servers?
IO (Israel O.) 0 Reputation points

2026-09-15T12:10:34.2566667+00:00

Hi @Hornblower409

Thanks for the reply and for linking that Reddit thread!

To clarify: no, there hasn't been any migration or host change here. We’re the same hosting provider, and these mailboxes have been on the exact same Linux/Plesk servers all along.

This behavior started popping up around November/December. It seems to affect pre-existing IMAP accounts on standard cPanel/Plesk environments—both for users who had the account working previously and for those who removed it during routine troubleshooting and couldn't re-add it.

What’s puzzling is that the webmail interface and all other mail clients (Classic Outlook, Thunderbird, iOS/Android Mail) authenticate without any issues using the exact same credentials. It only fails in New Outlook, and if we create a brand-new test mailbox on that same server, New Outlook connects right away.

Because we're an independent hosting provider managing third-party IMAP accounts rather than M365 tenants, we don't have a direct path to Microsoft Support to check or clear any cached settings on the cloud sync side. That’s why I posted here—and as you noted, it looks like we aren't the only ones running into this, given similar discussions over the past year (like Q&A threads #5491461 and #5881252).

It’s very possible it’s related to how cached settings or authentication states are handled on the cloud server side for these IMAP accounts. Hopefully, someone from the Microsoft team can take a look and see if there's something on their end that can be cleared or addressed.

Appreciate you jumping in on this!

Sincerely,

IO
Hornblower409 15,330 Reputation points

2026-09-15T13:40:20.92+00:00
@IO (Israel O.) said

Hopefully, someone from the Microsoft team can take a look and see if there's something on their end that can be cleared or addressed.

Microsoft Support does not monitor this forum.

we don't have a direct path to Microsoft Support to check or clear any cached settings on the cloud sync side.

How about having the affected users start the Support process?

If they have a Qualified MS 365 Subscription:
https://www.microsoft.com/en-us/microsoft-365/support

Otherwise free Chat:

Your problem here is that you need to get to the New Outlook support que and the best way to do that is from inside New Outlook, but they can't complete the New Outlook setup.

All I can suggest is that you give them a new test account that can connect, and then once they are in New Outlook they can start a Chat. Once in Chat they can tell the agent what is the account they are actually having problems with.

From the New Outlook main menu: Help -> ? Help

{This should open the Help side panel}

Search Help -> {Your problem} and press Enter

Still need help? [Yes]

If you get "We're here to help": Tell us your problem -> {Your problem, again} [Get Help]

[Sign In] {Use any Outlook/Microsoft account}

Choose a support option -> Chat with a support agent in your web browser

1 answer

Your answer

Hornblower409 15,330 Reputation points

2026-09-15T04:57:57.4733333+00:00

Are you saying that - You have an IMAP account that was added to New Outlook for Windows and was working OK. But you have changed the hosting service and Outlook keeps trying to connect to the old host?

If so, I think I'm working a similar problem on Reddit. Is this your issue?
[External Non-Microsoft Site] https://www.reddit.com/r/Outlook/comments/1wel696/new_outlook_changing_the_imapremoteserverinfo_is/

Have you brought in Microsoft Support to see if they can clear the cached settings on the cloud mail servers?
IO (Israel O.) 0 Reputation points

2026-09-15T12:10:34.2566667+00:00

Hi @Hornblower409

Thanks for the reply and for linking that Reddit thread!

To clarify: no, there hasn't been any migration or host change here. We’re the same hosting provider, and these mailboxes have been on the exact same Linux/Plesk servers all along.

This behavior started popping up around November/December. It seems to affect pre-existing IMAP accounts on standard cPanel/Plesk environments—both for users who had the account working previously and for those who removed it during routine troubleshooting and couldn't re-add it.

What’s puzzling is that the webmail interface and all other mail clients (Classic Outlook, Thunderbird, iOS/Android Mail) authenticate without any issues using the exact same credentials. It only fails in New Outlook, and if we create a brand-new test mailbox on that same server, New Outlook connects right away.

Because we're an independent hosting provider managing third-party IMAP accounts rather than M365 tenants, we don't have a direct path to Microsoft Support to check or clear any cached settings on the cloud sync side. That’s why I posted here—and as you noted, it looks like we aren't the only ones running into this, given similar discussions over the past year (like Q&A threads #5491461 and #5881252).

It’s very possible it’s related to how cached settings or authentication states are handled on the cloud server side for these IMAP accounts. Hopefully, someone from the Microsoft team can take a look and see if there's something on their end that can be cleared or addressed.

Appreciate you jumping in on this!

Sincerely,

IO
Hornblower409 15,330 Reputation points

2026-09-15T13:40:20.92+00:00

@IO (Israel O.) said

Hopefully, someone from the Microsoft team can take a look and see if there's something on their end that can be cleared or addressed.

Microsoft Support does not monitor this forum.

we don't have a direct path to Microsoft Support to check or clear any cached settings on the cloud sync side.

How about having the affected users start the Support process?

If they have a Qualified MS 365 Subscription:
https://www.microsoft.com/en-us/microsoft-365/support

Otherwise free Chat:

Your problem here is that you need to get to the New Outlook support que and the best way to do that is from inside New Outlook, but they can't complete the New Outlook setup.

All I can suggest is that you give them a new test account that can connect, and then once they are in New Outlook they can start a Chat. Once in Chat they can tell the agent what is the account they are actually having problems with.

From the New Outlook main menu: Help -> ? Help

{This should open the Help side panel}

Search Help -> {Your problem} and press Enter

Still need help? [Yes]

If you get "We're here to help": Tell us your problem -> {Your problem, again} [Get Help]

[Sign In] {Use any Outlook/Microsoft account}

Choose a support option -> Chat with a support agent in your web browser

Answer 1

IO (Israel O.) 0

Hi @Hornblower409

That in-app workaround using a temporary test mailbox to reach the New Outlook direct support queue is an absolute game-changer!

Since standard non-M365 IMAP clients can't open M365 admin tickets, using a working test profile inside New Outlook to initiate the internal support chat gives us a direct line to the desktop client engineering queue.

We are going to test this exact workflow on our end today with an affected test mailbox to get a case logged directly with their in-app support team. Really appreciate you sharing this path!

Sincerely,

IO

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