Microsoft has issued a warning regarding a significant bug in the Outlook Classic application that affects CPU performance. This issue, first identified in November 2024, can cause CPU usage to spike by up to 50% when users are simply composing an email or typing text. As of April 18, 2025, there is still no official patch available to address this problem.
The bug is specifically impacting users of Microsoft 365 who are utilizing Outlook on the Current Channel, Monthly Enterprise, or Insider Program versions, specifically those running version 2406 (build 17726.20126 and later). These channels frequently receive new features but can also experience instability and bugs during testing.
Users have reported that the CPU usage increases even when opening a new email compose window without typing any content, leading to significant resource consumption. In some cases, users observed that their devices’ cooling fans were operating at maximum capacity and reported abnormal heat levels. High-performance chips like the Intel Core i9-14900HX have been recorded at temperatures nearing 95 degrees Celsius during these light office tasks, which is unusual.
Microsoft has acknowledged that the issue relates to the Outlook word processor, although the precise cause remains unidentified. Users have attempted various troubleshooting methods, including disabling features such as spell check and graphics acceleration or turning off add-ins, but these actions have not mitigated the problem.
As a temporary solution while awaiting an official fix, Microsoft recommends users switch to the more stable Semi-Annual Enterprise Channel, which updates biannually and undergoes thorough testing. Individual users can adjust their update channels through the Command Prompt, while organizations are advised to utilize tools like Group Policy, Microsoft Intune, or the Office Deployment Tool for bulk updates.
This matter is under observation by Microsoft’s product development team, and feedback from the user community is being collected. Frustrated users have expressed their concerns on technical forums, including Microsoft Tech Community and Reddit, indicating that this issue has persisted for several months without a complete resolution.
