Microsoft Exchange Online Outage Blocks IMAP4 Mailbox Access Due to Authentication Code Conflict

Microsoft is working to resolve an Exchange Online service outage that intermittently prevents users from accessing their mailboxes via the Internet Message Access Protocol 4 (IMAP4). The incident, tracked under reference EX1215307 in the Microsoft 365 Admin Center, was first acknowledged on Wednesday, January 8, 2026 at 23:35 UTC.

The disruption has been classified as an incident in the admin center—a designation Microsoft typically reserves for critical service issues with noticeable user impact. While the company has not disclosed specific regional impacts or the total number of affected users, the incident classification signals significant disruption across the Exchange Online customer base.

Microsoft has confirmed that other connection methods remain unaffected, and users experiencing access issues may find success through retry attempts while the fix continues to deploy across the infrastructure.

Scope of Disruption

The outage specifically targets IMAP4 connectivity, leaving users who rely on third-party email clients unable to consistently access their Exchange Online mailboxes. Organizations heavily dependent on IMAP4 for email synchronization—particularly academic institutions, Linux environments, and businesses using non-Microsoft email applications—face the most significant operational impact.

Affected Services

• IMAP4 mailbox access to Exchange Online
• Third-party email client synchronization (Thunderbird, Evolution, Apple Mail via IMAP)
• Automated systems and scripts utilizing IMAP4 for email retrieval
• Cross-platform email applications configured for IMAP connectivity

Unaffected Services

• Outlook Web App (OWA) remains fully operational
• Outlook Desktop client via MAPI/Exchange Web Services
• Outlook Mobile applications
• Exchange ActiveSync connections
• POP3 and SMTP protocols
• Microsoft Teams integration with Exchange

Root Cause Identification

Microsoft has identified the root cause as a code conflict introduced during a recent IMAP deployment related to authentication support. This conflict created an authentication misconfiguration that manifests as intermittent mailbox access failures for users connecting via the IMAP4 protocol.

Official Microsoft Statement:

"A recent IMAP deployment related to authentication support contains a code conflict that introduced a configuration issue and is causing intermittent mailbox access issues for some impacted users."

Technical Context

The authentication misconfiguration affects the IMAP4 service layer responsible for validating user credentials and establishing mailbox sessions. When the misconfiguration triggers, affected users receive authentication failures or connection timeouts despite providing valid credentials.

The intermittent nature of the issue suggests the problematic code path is not universally invoked, likely depending on load balancer routing, specific mailbox server assignment, or timing-related factors in the authentication flow.

Remediation Approach

Microsoft has deployed a configuration change to address the underlying code conflict. The fix is being progressively rolled out across the Exchange Online infrastructure, with completion expected by the next scheduled service update.

Microsoft's Official Response

Microsoft acknowledged the incident within hours of initial user reports and has moved rapidly to identify and address the root cause. The company's engineering teams have already deployed remediation and are monitoring the rollout.

Key Communications:

Timestamp (UTC)	Update
January 8, 23:35	Incident EX1215307 formally acknowledged
January 8, 23:35+	Root cause identified: authentication code conflict
Ongoing	Configuration fix deployment in progress

Microsoft's Commitment:

"We're continuing to deploy the configuration change and expect this process to complete by our next scheduled update."

Incident Classification

The designation as a formal "incident" in the Microsoft 365 Admin Center indicates Microsoft's assessment that the issue produces noticeable user impact requiring priority attention. This classification typically triggers enhanced monitoring, accelerated engineering response, and structured customer communications.

Communication Channels

• Microsoft 365 Service Health Dashboard (Admin Center)
• @MSABORPS365 Twitter/X account for service alerts
• Direct admin notifications for affected tenants

Immediate Actions for End Users

1. Retry Connection Attempts: Microsoft confirms that retry attempts may successfully restore access due to the intermittent nature of the issue. Configure your email client to retry automatically or manually refresh the connection.

2. Use Alternative Access Methods: Access your mailbox through Outlook Web App at outlook.office.com for full functionality during the disruption.

3. Switch Protocols Temporarily: If your email client supports multiple protocols, consider temporarily configuring Exchange ActiveSync or switching to the Outlook desktop application.

4. Monitor for Resolution: The fix is actively deploying—periodic connection attempts will eventually succeed as the configuration change propagates.

Guidance for IT Administrators

• Track Incident EX1215307: Monitor the Microsoft 365 Service Health Dashboard for real-time updates and estimated resolution timelines.

• Communicate with Affected Users: Identify IMAP4-dependent users in your organization and provide workaround guidance proactively.

• Prepare for Service Restoration: No user-side configuration changes are required once the fix completes—IMAP4 access should automatically resume.

• Document Impact: Log affected users and duration for internal incident tracking and potential service credit requests.

Current Status

Identified and Mitigating — Microsoft has deployed a configuration fix and is actively rolling it out across Exchange Online infrastructure. Full resolution is expected with the next scheduled service update.

Pattern of Recent Incidents

This IMAP4 disruption continues a pattern of Exchange Online incidents over recent months:

Date	Incident	Root Cause
January 2026	IMAP4 access blocked	Authentication code conflict
November 2025	Classic Outlook mailbox access	Infrastructure connectivity
October 2025 (late)	Azure/M365 DNS outage	Network configuration
October 2025 (early)	SSO/MFA authentication	Multi-Factor Authentication issues

Preliminary Lessons

1. Authentication Deployments Carry Risk: Changes to authentication infrastructure require enhanced testing protocols given their potential for widespread service impact.

2. Protocol-Specific Testing: Deployments affecting individual protocols (IMAP4, POP3, etc.) should include protocol-specific validation to catch issues before production rollout.

3. Rollback Readiness: The ability to quickly revert authentication changes limits blast radius when code conflicts emerge.

Post-Incident Report

Microsoft typically publishes a Post-Incident Report (PIR) within 5 business days of incident resolution. This section will be updated with detailed root cause analysis and preventive measures once available.