Microsoft has fixed a known issue causing desktop and mobile email clients to fail to connect when using Outlook.com accounts.

"The Outlook.com Team implemented service changes on January 31, 2024 to address the connection issues with unexpected authentication prompts," Microsoft said.

"For Outlook 2013 and Outlook 2016, if you are still seeing authentication prompts, please ensure you've enabled two step verification and create an app password. Use the app password in place of your normal password when Outlook prompts for authentication."

More details on how to use app passwords with apps without two-step verification support can be found in this support document.

Customers affected by these sign-in problems started sharing reports on Microsoft's community platforms and other social networks like Reddit last Tuesday.

Impacted users saw pop-ups requiring them to enter their account passwords and giving them a "Remember my credentials" option. However, even for those who entered the correct credentials, the login window would still pop up as the app tried connecting to the Outlook.com account.

"Since starting around 1/23/24 users have reported issues connecting with Outlook 2013, Outlook 2016, Outlook for Microsoft 365, Thunderbird, and mobile email apps when connecting with POP, IMAP, and Exchange connections," Microsoft said when first acknowledging the issue.

"Some users have reported that signing in with an App Password works to connect but later goes back into a disconnected state."

Fixes for other Outlook issues

Outlook.com users have been experiencing other issues in recent months that have prevented them from searching for email content or caused their email clients to crash.

Two months ago, the Outlook team fixed another known issue that caused Outlook Desktop clients to crash while sending emails from Outlook.com accounts in Version 2311 Build 17029.20068.

After resolving the bug, Microsoft advised users who disabled auto-updates to click "Update Now" under File > Office Account > Update Options to get the fix.

Redmond also resolved an issue in July that was causing 401 exception errors for Outlook.com users and breaking email searches.

Before this, in June, multiple Microsoft services, including Outlook.com, OneDrive, and the Azure Portal, were disrupted by DDoS attacks later claimed by a threat actor known as Anonymous Sudan, believed to have Russian links.