At least five different hacking groups, including one from China, are currently attacking vulnerabilities in Microsoft's email servers -- described by the US government as "widespread domestic and international exploitation" that may affect hundreds of thousands of victims globally.

While Microsoft has already revealed that a Chinese government-linked hacking group known as Hafnium is targeting its on-premises 'Exchange Server' software, the MIT Technology Review now reports that at least "four other distinct hacking groups are now attacking critical flaws in Microsoft's email software".