Is it Patch Blues-day for Outlook? Microsoft's email client breaks worldwide, leaves everyone stumped

Updated Microsoft's desktop email client Outlook has stopped working worldwide for countless users, whether they are using it with an on-premises Exchange server or with the Office 365 cloud.

When you try to start the software on Windows, it immediately crashes with the error code 0xc0000005. "Microsoft has borked Outlook," one Reg reader told us. "Thousands of users worldwide are now experiencing this."

It appears Outlook 2019 and 2016 are affected, whether they are using a cloud-hosted Office 365 service, Outlook.com, or on-prem Exchange. Monitoring at DownDetector shows that, indeed, America and Europe, at least, are experiencing failures right now, starting around 9am PT (1600 UTC).

Microsoft did not immediately respond to a request for comment, though the official Microsoft 365 cloud status Twitter feed acknowledged the breakdown, and pointed toward yesterday's July Patch Tuesday update as a possible cause for the borkage:

We're investigating whether a recently deployed update could be the source of this issue. As a workaround, users can utilize Outlook on the web or their mobile clients. Additional details can be found in the admin center under EX218604 and OL218603.

— Microsoft 365 Status (@MSFT365Status) July 15, 2020

As you can see, we're told the web and mobile Outlook clients are not affected, and that Redmond's engineers are on the case. On its Office 365 status page, Microsoft warned as we published this story that it may take another couple of hours to correct the blunder, for Outlook.com users at least:

Folks who want to avoid the crashes are unofficially advised to roll back their Outlook installations to a known-working June 24 version, and this can be done via the Windows command line:

cd “\Program Files\Common Files\microsoft shared\ClickToRun”

officec2rclient.exe /update user updatetoversion=16.0.12827.20470

We'll let you know when we know more. ®

Updated at 0100 UTC, July 16

Microsoft says it resolved the issue at 2235 UTC by pushing out a fix to users, roughly six hours after trouble started.

It noted on its Outlook.com status page: "We've confirmed that the issue has been successfully resolved after extended monitoring of our telemetry. If users are continuing to see impact, please restart your Outlook client for the changes to take effect."

A spokesperson for Microsoft also told us: "Our teams have begun to roll out a mitigation for an issue affecting user access to Outlook." The spokesperson added that Wednesday's breakage was not a result of the July Windows Update batch as previously speculated. We've asked the Windows giant to elaborate and pinpoint what exactly caused the breakdown.

In the meantime, it appears the crashes were linked to the Office Click-to-Run component; rolling that back to an earlier version allowed the software to run as normal. Undoing the Patch Tuesday security patches may have had no effect, so if you've uninstalled them, you should probably reapply them because they fix up a remote-code-execution hole in Outlook.