Use S/MIME to send encrypted messages in an Exchange environment in iOS – Apple Support

iOS offers support for S/MIME so that you can send encrypted email messages. 

— more: support.apple.com/en-us/HT202345

E-mail by default is infringing your privacy. No law for privacy applies, like there is with regular (non-digital) mail.

Exchange provides an environment to support a degree of privacy on an iPhone or iPad: “In iOS 13.4 and later, when you reply or forward a message, the encryption state of your message will match the state of the incoming message rather than your system default setting.“