I have an internal Mailcow installation which works really well except for the setup in Outlook which asks for authentication twice. Once during the normal setup as expected, but when it starts testing the connectivity it will ask for the password in a Windows (Basic) Auth Popup as seen in the screenshot. Once i enter the password again Outlook will successfully autodiscover IMAP.

Thunderbird works without any issue, so I am guessing this is related to Outlook only.

Any ideas what could be causing this?

If you are using Outlook you should switch to ActiveSync, so all your contacts and the calendar are synced as well. You have to create your ActiveSync account using the Windows Control Panel, as you can’t create it from within Outlook unfortunately…

Control Panel -> Mail (Microsoft Outlook) (32 Bit) -> Email Accounts -> New –> Manual configuration -> Exchange ActiveSync

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