Just for those rare corporate users out there, neither Edison Mail nor Outlook for iOS supports encrypted emails (S/MIME). They can encrypt the communication to the Mail server just like going to a secure Website, but the email itself cannot be encrypted nor digitally signed to verify its authenticity. The process of using PGP encryption is similar with these apps, however, as it is to Apple's Mail app - you use a third party app like oPenPG to basically copy and paste between Mail and a third-party app which can handle the encryption and decryption.
Apps like R2Mail2 on Android support both PGP and S/MIME natively for a seamless experience, plus it has a beautiful dark mode that's so easy on the eyes, and can color-code each account in a unified mailbox. Basically, after using Android for Mail and phone calls out of frustration for a while while I tried desperately to fix my iPhone, I can tell you my next phone will be an LG, not an Apple.
And if you do not encrypt your email, you should seriously consider starting. It's free, and while not as easy as flipping a switch, it's fairly straightforward and totally transparent once it's set up.