Microsoft has fixed an issue causing some Microsoft 365 users' Outlook desktop clients to stop connecting to email servers via Exchange ActiveSync.

Exchange ActiveSync (EAS) is a synchronization protocol used by Microsoft Exchange to allow users to access their email, calendar, contacts, and tasks.

EAS uses HTTP and XML to communicate and synchronize data between the Exchange server and mobile devices. By default, EAS is enabled on new user mailboxes, and disabling it can prevent users from synchronizing their mailboxes with mobile devices.

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

The known issue only impacts Outlook for Microsoft 365 users who have updated their clients to Version 2401 Build 17231.20182.

Microsoft says the known issue has been resolved in Version 2402 Build 17328.20068 and higher and asks customers to install the update by clicking "Update Now" under File > Office Account > Update Options.

Before resolving the issue, Redmond also provided a temporary fix for affected users to revive their Outlook desktop clients, which required 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 didn't have this sync issue.

"If you worked around the issue by reverting to the prior build, re-enable updates by selecting File > Office Account > Update Options > Enable Updates," the company said today.

Microsoft is also currently investigating an issue that triggers security alerts in Outlook when attempting to open .ICS calendar files after installing December 2023 Patch Tuesday Office security updates.

Last month, Microsoft fixed another known Outlook issue, which caused connection problems for desktop and mobile email clients using Outlook.com accounts.

In December, Microsoft addressed two additional bugs: one caused email-sending issues for users with many folders, while the other caused Outlook to crash while sending emails from Outlook.com accounts.