New Security Layer added in O365

SMTP MTA Strict Transport Security (MTA -STS)

Uses TLS to ward-off man-in-the-middle attacks 

Microsoft  uses SMTP - an old e-mail protocol that permits text exchanges without much security

What problem does the MTA-STS solve?

Expired TLS certificates Problems with 3rd Party Certificates etc. 

Your email data remains safe during transit 

"There's nothing admins need to do to leverage it"  says Microsoft

The recipient and the sender both need to have MTA-STS in place

To get many such benefits of M365