​​Microsoft has shared a temporary fix for a known issue preventing Microsoft 365 customers from signing in or adding Gmail accounts using classic Outlook.

When attempting to synchronize Gmail accounts with their Outlook profile or add a new Gmail account, affected users may encounter the following errors: “This browser or app may not be secure” and “We weren’t able to set up your Google account with IMAP. You may need to make changes to your Gmail settings.”

Users are also advised to attempt signing into their Gmail accounts using a different browser if they encounter login problems.

“This error above can also occur if you do not have IMAP enabled for your Gmail account. Google is in the process of removing that control, but it is still there in account settings for some users,” Redmond explained in a recently published support document.

Google explains that the option to toggle IMAP was removed in June 2024 because IMAP access is always enabled in Gmail.

The company says a fix that changes the sign-in procedure to open another browser window is already available and is rolling out to Office Insiders in Current Channel Preview and Office Insider.

​Until generally available, Microsoft advises impacted customers to switch to Office Insider to get the fix in advance by adding or creating an insiderslabbehavior REG_DWORD entry set to ‘1’ under HKEY_CURRENT_USERSoftwarePoliciesMicrosoftoffice16.0common.

They can also execute a registry key that allows them to switch directly to Microsoft 365 Insider after restarting Outlook by clicking ‘Update Now’ under File > Office Account > Update Options.

In April, Redmond also warned customers that Gmail blocks and marks some Outlook.com emails as spam, an issue impacting users with Outlook.com country domains.

At the time, Google suggested that Microsoft review new bulk sender guidelines to improve Outlook email deliverability to Gmail accounts.

Since April, Google has been blocking emails sent by bulk senders who don’t follow stricter spam thresholds to better protect its users against phishing and spam.