Outlook.com: IMAP-XOAUTH2 login fails – some mail clients affected

Mail[German]Microsoft seems to have recently changed the authentication for the IMAP protocol on its Outlook.com email service. It is no longer possible to use XOAUTH2, so some mail clients refuse to cooperate. Here is a brief overview of the situation.


What is XOAUTH2?

XOAUTH2 is an authentication method used as the default for logging into IMAP mailboxes in GMail, among others. According to this post, XOAUTH2 includes the OAuth 2.0 standard and additional non-standard SASL binding for OAuth. This post covers authentication using XOAuth2 for IMAP and SMTP mailboxes.

Outlook.com does not support XOAUTH2

I became aware of the issue with this German blog post from Martin Geuss. Developer Robert Virkus also addressed the issue on Microsoft Answers in this forum post. Robert is a developer of an email client (see) and found out that logging in to the IMAP mailboxes of outlook.com does not work anymore since September 24, 2021 using XOAUTH2. The server suddenly returns the error message:

a0 AUTHENTICATE XOAUTH2 (base64 code scrambled)

on corresponding message of the e-mail clients. So Microsoft must have changed something, because the app was able to access the IMAP server with XOAUTH2 before the above mentioned deadline. The app still works with a Google GMail account when using XOAUTH2. The bug not only affects Robert with its email client, but also other email programs and apps like eM Client, Aqua Mail, Fair Email, etc.

Microsoft confirms the problem

On the Microsoft Answers forum, a Microsoft moderator confirmed the problem to Robert.


We're completely aware of the issue going on with the Microsoft accounts that won't connect using a third party mail app. Please be advised that we already consulted this to our internal teams and we'll get back you as soon as we get an updates from them.

Microsoft was already aware of the problem and had passed it on to the internal teams. From there came the feedback that there was a corresponding discussion on the topic here. The advice was to set up the mail account in the relevant eM client not as an IMAP account, but temporarily as an Exchange account. Then it should work, but has the problem that various clients do not support it. Those affected can only wait and hope that Microsoft fixes this soon.

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