MTA-STS is a security standard for secure e-mail delivery. E-mail servers that send inbound e-mail to the Criminal Justice Hub will be able to detect that our e-mail server supports SMTP-over-TLS via STARTTLS (also known as Opportunistic TLS) before opening the actual connection.
In case the sending e-mail server is not able to initiate a secure connection, it will end the connection to enforce transport layer encryption. This mitigates Man-in-the-middle DNS and SMTP downgrade attacks that would allow an attacker to read or manipulate e-mail in transit.