We use MTA-STS (for inbound AND outbound) with our mode set to enforce[1], to require senders to communicate with us only using TLS encrypted sockets. There is no legal precedence currently requiring software services to implement backdoors.
Also - you should note that we largely operate in-memory and don't store to disk any information or logs (unless essential, e.g. IMAP storage, or if they are error logs). We have all of this in our privacy policy and terms on our website. We are extremely transparent.