Microsoft warned Outlook for Microsoft 365 users that clients might have issues connecting to email servers via Exchange ActiveSync after a January update.

Exchange ActiveSync (EAS) is an Exchange synchronization protocol using HTTP and XML to let users access their email, calendar, contacts, and tasks.

EAS is enabled by default on new user mailboxes, and disabling it prevents users from synchronizing their mailboxes with mobile devices.

"After updating to Version 2401 Build 17231.20182 Outlook stops connecting when using the Exchange ActiveSync (EAS) protocol," Microsoft said.

"We have to use Activesync in order to connect to our cloud-hosted email server. Other syncing may not be impacted," one impacted user said.

While the Outlook Team has yet to provide an explanation for this syncing issue, it's currently investigating and will provide a fix as soon as a solution is found. 

Workaround available

Until this EAS syncing issue is resolved, Microsoft provides a temporary fix for affected users to revive their Outlook desktop clients.

This requires reverting Office M365 Click-to-Run installations to an unaffected Office build (the suggested build is Version 2312 Build 17126.20132) or switching to an Office Channel that does not have this sync issue.

To apply this workaround, you will have to go through the following procedure:

1. Disable updates temporarily to ensure Outlook doesn't automatically update to the broken version from File > Office Account> Update Options>Disable Updates.
2. Exit all Office applications, Select Start, type cmd in the Search box, right-click Command Prompt (or cmd.exe), and then select Run as administrator.
3. In the command prompt dialog window, type the following two command lines, pressing Enter after each command line:
   • cd %programfiles%\Common Files\Microsoft Shared\ClickToRun
   • officec2rclient.exe /update user updatetoversion=16.0.17126.20132
4. When the repair is done, open Outlook and select File > Office Account.
5. After the version revert process is finished, launch Outlook to check if it's operating normally again.

Microsoft is also investigating an issue triggering Outlook security alerts when trying to open .ICS calendar files after December 2023 Patch Tuesday Office security updates are installed.

Redmond fixed another known Outlook issue this month that was causing connection issues for desktop and mobile email clients when using Outlook.com accounts. 

In December, Microsoft addressed two other bugs, one behind email sending issues for users with lots of folders and one causing Outlook to crash when sending emails from Outlook.com accounts.