Microsoft is investigating an Exchange Online service outage that is preventing customers from accessing their mailboxes using the classic Outlook desktop client.

Microsoft first acknowledged the incident (tracked under EX1189820) at 09:57 AM UTC and, according to user reports on the DownDetector outage monitoring platform, it’s also causing server connection and login issues.

“Users may be unable to connect to their Exchange Online mailbox in the classic Outlook experience. We’re actively investigating the root cause of the connectivity failures,” Microsoft said.

“Impact is specific to users located in the Asia Pacific and North America regions attempting to connect to their Exchange Online mailbox using the classic Outlook experience.”

While Redmond has yet to share how many users are impacted by this ongoing issue, it has been tagged as an incident in the admin center, a label usually applied to service problems with significant user impact.

As a workaround, Microsoft advises affected customers to access their mailboxes via Outlook on the Web.

Classic Outlook search issues

Microsoft now analyzes the root cause of these connectivity failures, as well as a separate incident (EX1189768) acknowledged on Thursday morning, at 05:05 AM UTC, that is causing search issues for classic Outlook users.

“Impact is specific to some users attempting to search in the classic Outlook desktop client. We’re analyzing service-side logs to identify the root cause and develop a mitigation plan,” it added.

This follows a major DNS outage that impacted Azure and Microsoft 365 services in late October, preventing users from logging into company networks and accessing multiple services and platforms.

In early October, it mitigated another outage blocking customers’ access to Microsoft Teams, Exchange Online, and the admin center via Microsoft Entra single sign-on (SSO) authentication due to Multi-Factor Authentication (MFA) issues.

One day later, Microsoft resolved another incident caused by an Azure Front Door content delivery network (CDN) issue that took down Microsoft 365 services for customers across Europe, Africa, and the Middle East.

This is a developing story…