Update on April 25, 2025: As of yesterday, Microsoft has confirmed the issue and implemented a fix, but that fix won’t actually roll out until “early May” (Beta Channel), “mid May” (Current Channel Preview), or “late May” (Current Channel). If you absolutely can’t wait that long for the update, you can revert to version 2405 of classic Outlook, but you’ll be giving up several important security fixes.
Original story from April 16, 2025: If you’re still using Microsoft’s classic Outlook app for your emailing needs, you may have noticed that your computer hardware behaves a bit strangely when using the app—most notably when typing.
Earlier this week, Microsoft issued a warning and confirmed that the classic Outlook app can cause your computer’s CPU usage to skyrocket. Processor usage can spike by 30 to 50 percent when writing an email, and also increase your computer’s power consumption. It’s implied that this is a bug, but the root cause isn’t yet known.
The company says there isn’t yet a direct solution to the problem, but notes that the issue only affects versions of classic Outlook on the Current Channel, Monthly Enterprise Channel, and Insider channels, after updating to Version 2406 Build 17726.20126 or later.
If this issue is causing you significant trouble, you can work around it by switching your classic Outlook app to the Semi Annual Channel. Instructions on how to do that can be found in Microsoft’s warning, either using the Office Deployment Tool or a Windows registry change. You can also read ongoing discussion of the issue in this Microsoft forum thread.