People who use Gmail for the email processes have been told of a data security attack on the system.
Google has identified the breach, noting that the incident looks to have been orchestrated by hackers in China.
Hundreds of users have been caught up in the attack, including government officials in the US, in a breach that circulated through a phishing scheme.
This method was used to access the accounts of users and then monitor their emails and change any forwarding settings that were set up on there.
As a result of the breach, the Pentagon has said that it is prepared to retaliate with force against nations that compromise the US' systems.
However, a Chinese foreign ministry spokesman named Hong Lei pointed out: "Hacking attacks are an international issue. China is also a victim."
Recently, George Schlossnagle, the president and chief executive of Message Systems, told those worried about their email security to be aware of "spear-phishing", which has been affecting firms for at least the past year.