In late June Google will be releasing “confidential mode” in Gmail. Here’s a few key things to keep in mind

• this only prevents accidental sharing of an email, people can still take a picture or screen shot of the content or attachments
• this feature is meant to protect personal or sensitive information but not regulated or legally governed data
• any regulated data (SSNs, credit card numbers, bank information) should still never be sent via email. When in doubt consult the Data Classification Policy and the Regulated Data Storage Chart.

More detail is found here