Outlook Classic suddenly asking for credentials (Windows security popup)

Question

Outlook Classic suddenly asking for credentials (Windows security popup)

Christine Devaux 0

I have a new user. We use Exchange on premises and Outlook classic. When the user is connected to our network in the office of throught the VPN, no problem at all but once he's outside there is a popup coming up (copied below), which is very unusual and his password is not accepted, the popup reappears a few seconds later. Once the user connect to the VPN, the popup disappear and Outlook works perfectly.
I already checked if there was any warning in the Account, logged out and in again to Office account, removed the account in Mail in the Control Panel and reconnected him but still the same, disconnecting from our internal network makes this popup appear again.
It's the only user with that problem. Any idea what could go wrong?
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Answer 1

Jade Ng 14,780 Microsoft External Staff Moderator

Dear @Christine Devaux,

Based on your description, the issue does not appear to be related to an incorrect password, as Outlook functions normally whenever the user is connected to the corporate network through VPN. This suggests the issue may be related to how Outlook is connecting to the on-premises Exchange environment when the user is outside the internal network.

To help narrow down the cause, could you please confirm the following:

Does the affected user have access to Outlook on the web (OWA) when outside the network and not connected to VPN?
If the mailbox is configured on another computer outside the network, does the same behavior occur?
In Outlook, does the status show "Disconnected", "Trying to connect", or "Need Password" when the prompt appears?
What version of Exchange Server is being used?
Have there been any recent changes to certificates, Exchange configuration, firewall, reverse proxy, or Autodiscover settings?

As an initial troubleshooting step, I recommend testing Outlook Autodiscover and clearing any cached credentials:

Close Outlook.
Open Credential Manager and remove any Outlook, Office, or Exchange-related credentials.
Reopen Outlook and test again while off the VPN.

I look forward to your update. The additional information should help me identify the next troubleshooting steps.

Thank you very much for your valuable time and your cooperation

Christine Devaux 0 Reputation points

2026-08-06T07:41:04.63+00:00

Hello @Jade Ng
Access via OWA is OK, no problem at all. I can unfortunately not test the mailbox on another computer because my user is now abroad and I need him to give the authentication codes.
I don't remember exactly what the Outlook status was unfortunately.
Exchange Server is the latest SE edition (Exchange Server 19 SU14 if I am right)
No recent changes, just the usual updates made to our server and all Office updates of course.

I will try what you recommend as soon as I get hold of the laptop (which may take a few days)

Thanks for your help!
Jade Ng 14,780 Reputation points Microsoft External Staff Moderator

2026-08-07T04:54:06.1033333+00:00

Dear Christine Devaux,

Thank you for your response.

It's helpful to know that OWA is working normally, which suggests the issue may be isolated to the Outlook client on the affected device.

I understand the user is currently abroad and cannot test on another computer at this time. Once you have access to the laptop, please try the recommended steps and let me know the results.

I appreciate your patience and look forward to your update.
Jade Ng 14,780 Reputation points Microsoft External Staff Moderator

2026-08-09T23:27:39.4666667+00:00

Dear Christine Devaux,

I hope everything is going well on your end.

I’m following up on the support thread we've been working through together. I just wanted to check in and see if you have any updates or additional information you’d like to share regarding your situation.

Looking forward to your response!
Christine Devaux 0 Reputation points

2026-08-10T05:36:46.9433333+00:00

@Jade Ng Hello! Unfortunately no additional feedback as the user is not coming back to the office this week and I will not have access to the laptop before mid-September after my own holidays :-(
I will come back to this thread as soon as I can test the proposed solutions - thanks again!
Jade Ng 14,780 Reputation points Microsoft External Staff Moderator

2026-08-10T06:09:56.28+00:00

Dear Christine Devaux,

Thank you for the update, and no worries at all.

I completely understand the situation. Enjoy your holiday, and please don't feel any pressure to test the suggested steps until you have access to the laptop again.

When you're back and able to continue troubleshooting, simply reply to this thread with the results, and I'll be happy to pick up from where we left off.

In the meantime, I hope you and your colleague have a great break. Thank you for keeping me informed, and I look forward to hearing from you when you're ready to continue.