Consider requiring TLS for inbound and outbound mails
More or less all mail traffic is encrypted today. See e.g: https://transparencyreport.google.com/safer-email/overview.
Basically setting smtp_tls_security_level = encrypt and smtpd_tls_security_level = encrypt.
Before doing this we should run the numbers (inspiration in !865 (merged)) and perhaps send a mail to ADP.