Encrypted email isn’t perhaps as straightforward as other encrypted communication Apps.
For email, you can only exchange encrypted email with other contacts that use the same encryption platform. While iOS/iPadOS supports S/MIME, requiring SSL Certificates, this is only useful where everyone else with whom you communicate via email also uses S/MIME.
There are other encrypted email standards - and many require an appropriate email client if you are to be able to use them - such as PGP.
There are also proprietary encrypted email solutions that try to solve the interoperability problem that exists when trying to send encrypted email to someone that doesn’t have access to the same encrypted email system - such as Virtru.
In summary, without you having a much better understanding of what you are attempting to achieve - and perhaps importantly with whom - providing coherent and specific advice will be difficult. Suffice to say at this point - unless you are likely to be using a corporate email system that supports S/MIME, this is perhaps unlikely to be a viable solution for your need.