If you are looking to encrypt and send secure emails in iOS, iPGMail is the most full-featured and well-reviewed of the PGP apps on the App Store. Here is how it works.
Without email encryption, your emails can be spied on by any interested observer. This shows how you can encrypt your emails on macOS and send encrypted emails with any email client.
PGP is the gold standard for encrypted communication and has been used by everyone from nuclear activists to criminals. This article shows how it works and how you can get started.