SecureMyEmail
8.2#1 in email encryption softwareFrom $29.99 a yearFree planAndroidiOSWebmacOS
VisitSecureMyEmail suits people who want to encrypt email while keeping an existing provider, especially those using IMAP or one of its built-in services. It also fits individuals who need up to eight addresses or organizations seeking consolidated billing and support after a trial.
You want what its maker publishes above — check its page for the details.