Gmail's spam and malware filters are among the best in the industry and block 99.9% of threats from reaching inboxes, according to Google. That doesn't mean there isn't room for improvement, as 0.1% can scale up to quite a substantial absolute number considering Gmail's big user base. Google has identified that malware in attachments (Office documents, in particular) tends to be dangerous due to its ever-changing nature, so it has developed an improved malware scanner that utilizes machine learning to stay ahead of the curve.