Phishing (fraudulent attempt to obtain sensitive information such as usernames, passwords, and credit card details) is one of the most common e-mail based attacks used by cybercriminals to gain access to corporate information systems. In Office 365 the admin can create rules to prefix subject line with the text [EXTERNAL] when emails are received from
senders outside the company.
I think this is a cool thing to help people from getting phished. Earlier it was not very easy for a normal users to identify Email spoofing (creation of email messages with a forged sender address) which leads to phishing attacks. Way to go!
RaviSankar's Blog 