​Microsoft is working to resolve a known issue that causes an anti-spam service to mistakenly block Exchange Online and Microsoft Teams users from opening URLs and quarantine some of their emails.

In a service alert seen by BleepingComputer, the company stated that the issue is caused by the anti-spam engine incorrectly tagging URLs contained within other URLs as potentially malicious, which has also led to some emails being quarantined.

The issues began impacting Exchange Online and Microsoft Teams users on September 5th, when Redmond said that admins might see alerts titled "A potentially malicious URL click was detected involving one user," even though the URLs had already been confirmed as safe.

"We've identified over 6,000 URLs that are affected and are working to unblock them before replaying messages to recover any messages or URLs that were incorrectly flagged," Microsoft said the day it discovered the bug.

"Redmond's engineers have deployed a fix that addresses these problems by ensuring that the syncs no longer enter the quarantine state, after a previous configuration change that would've changed the configured delay interval to one hour wasn't successful."

While Microsoft engineers have partially resolved these false positive issues, they are still working to address the impact caused by more URLs being disabled by its faulty anti-spam models.

"We've identified a new subset of URLs that are impacted and we're working to address the new set and any residual impacted messages. We are confident that a majority of the impact has been resolved, and we're actively addressing lingering impact while we perform our root cause analysis," the company added in a September 8th update.

Although the company has yet to disclose the number of customers or the regions affected by these ongoing anti-spam problems, this service issue has been classified as an incident, which usually involves noticeable user impact.

Microsoft has addressed similar issues since the start of the year, resulting in emails being incorrectly tagged as spam or quarantined. For instance, in May, Microsoft resolved another issue causing a machine learning model to incorrectly flag emails from Gmail accounts as spam in Exchange Online.

Redmond fixed another machine-learning bug that mistakenly flagged Adobe emails in Exchange Online as spam one month earlier, as well as an Exchange Online false positive that caused anti-spam systems to incorrectly quarantine some users' emails in March.