Notes 
#1 
Bring email encryption straight into your browser. We recommend using Firefox for this. It can be used to encrypt and sign electronic messages, including attached files,…
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#2 
GNOME Evolution is a Linux personal information manager combining email, calendar, contacts, and tasks in one application. It uses standard protocols (IMAP, SMTP, CalDAV, CardDAV) and supports native OpenPGP and S/MIME encryption without plugins.
#3 
Kontact is the KDE personal information management suite that combines KMail (email), KOrganizer (calendar), KAddressBook, and a feed reader into a single unified window. It runs on Linux and supports IMAP, POP3, OpenPGP, and S/MIME natively.
#4 
NeoMutt is a command-line email client for Linux and macOS, forked from Mutt with a large collection of patches and added features. It supports IMAP, POP3, SMTP, OpenPGP, S/MIME, Notmuch search integration, and is highly configurable via a plain-text config file.
#5 
Mozilla Thunderbird is a email client, personal information manager, news client, RSS and chat client.
#6 
K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email. Designed for both novice and power users and offers lots of features…
#7 
Apple Mail is the built-in email client on macOS and iOS, supporting IMAP, SMTP, and S/MIME. Mail Privacy Protection hides the user's IP address and loads remote content via Apple proxy servers to block tracking pixels.