Gmail will become the first major email provider to follow the new SMTP MTA Strict Transport Security (MTA-STS) RFC 8461 and SMTP TLS Reporting RFC 8460 internet standards. Gmail will honor the new standard, initially launching in Beta on April 10th 2019. We hope many other email providers will soon adopt these new standards that make email communications more secure. G Suite admins can use the G Suite Updates blog to see what MTA-STs means for G Suite domains.”]
Source: https://security.googleblog.com/2019/04/gmail-making-email-more-secure-with-mta.html