Instead of using third party app for your encryption, I will show you a method to get a s/mime certificate to encrypt your emails and sign them.
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https://www.entrust.com/knowledgebase/ssl/how-do-i-install-my-secure-email-certificate-s-mime-on-ios-apple-iphone-ipad-ipod-touch-ios-5-or-later
https://formsmarts.com/iphone-smime-encrypted-email
https://extrassl.actalis.it/portal/uapub/freemail?lang=en
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https://www.entrust.com/knowledgebase/ssl/how-do-i-install-my-secure-email-certificate-s-mime-on-ios-apple-iphone-ipad-ipod-touch-ios-5-or-later
https://formsmarts.com/iphone-smime-encrypted-email
https://extrassl.actalis.it/portal/uapub/freemail?lang=en
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