Claws Mail is a GTK+ based, lightweight, and fast e-mail client and newsreader. Supports POP3, APOP, IMAP, SMTP, SMTP AUTH, NNTP, LDAP, multiple accounts, spell-checking, addressbook, SSL, GPG, filtering, i18n, & more. It is further extended by plugins. Lots of extra functionality, like an RSS aggregator, calendar, or laptop LED handling, are provided by extra plugins. 

Claws Mail - Email Client and News Reader Based on GTK+

Mailspring is built on the modern web with Electron, React, and Flux. It is designed to be extensible, so it's easy to create new experiences and workflows around email. 
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Mailspring comes packed with powerful features like Unified Inbox, Snooze, Send Later, Mail Rules, Templates and more. Mailspring Pro, which you can unlock with a monthly subscription, adds even more features for people who send a ton of email: link tracking, read receipts, mailbox analytics, contact and company profiles. All of these features run in the client - Mailspring does not send your email credentials to the cloud. 
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You can have unified inbox as it supports every major mail provider—Gmail, iCloud, Office 365, Outlook.com, Yahoo!, and IMAP/SMTP—so you have a single, streamlined command center for all your messages. 

Mailspring is built on the modern web with Electron, React, and Flux. It is designed to be extensible, so it's easy to create new experiences and workflows around email. 

Mailspring - A beautiful, fast Webmail

FairEmail is easy to setup and works with virtually all email providers, including Gmail, Outlook and Yahoo. It supports Unlimited accounts, Unlimited email addresses,Unified inbox, Offline storage and operations, Conversation threading, Encryption, Anti Phishing, Automatically recognize and disable tracking images and lot more.

FairEmail - Privacy friendly email app for Android

Thunderbird is most popular simple e-mail, newsgroup and news feed client. It is comparable to Microsoft outlook. It supports features such as Firefox's tabbed browsing, Email search, Message archive, Spam filtering, Message grouping, SSL, TLS, POP3 and IMAP support and lot more.

Thunderbird - Email Client

The SeaMonkey project is a community effort to develop the SeaMonkey all-in-one internet application suite. It contains an Internet browser, email & newsgroup client with an included web feed reader, HTML editor, IRC chat and web development tools. Under the hood, SeaMonkey uses much of the same Mozilla source code which powers such successful siblings as Firefox and Thunderbird. 

SeaMonkey - All-in-one internet application suite

Hastymail is a fast, secure, rfc-compliant, cross-platform IMAP/SMTP client application written in PHP providing a clean web interface for sending and reading E-mail. It is compatible with PDAs, phones, text browsers, and all mainstream browsers. The ability to use a database for storing user settings, contacts, and plugin data, with support for both Postgresql and Mysql. 

Hastymail - IMAP/SMTP client in PHP

K-9 Mail is an open-source email client for Android.
<ul>
  <li>Push IMAP support</li>
  <li>WebDAV support</li>
  <li>Attachment saving</li>
  <li>Attach any file on the SD card type when <a href="http://www.openintents.org/filemanager/">OI File Manager</a> is installed</li>
  <li>BCC to self</li>
  <li>Signatures</li>
  <li>Message flagging</li>
  <li>Per account notification</li>
  <li>Choice of black or white theme</li>
  <li>Multiple identities</li>
  <li>Message list sorting</li>
  <li>Keyboard shortcuts</li>
  <li>Many performance enhancements</li>
  <li><a href="https://k9mail.github.io/documentation/security/pgpmime.html">GPG &amp; PGP/MIME support</a> using OpenKeychain (formerly APG)</li>
  <li>Emoji support</li>
  <li>Pinch-to-zoom</li>
  <li>Mail on SD</li>
</ul>

K-9 Mail is an open-source email client for Android.

K-9 Mail – Advanced Email for Android

Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client. It provides intuitive user-interface, Powerful filtering and search, Junk mail control, Security features (GnuPG, SSL/TLSv1). Sylpheed supports not only the major protocols such as POP3, IMAP4rev1 and SMTP, but also NNTP (NetNews). IPv6, which is the next-generation Internet protocol, is also supported by default.

Sylpheed - Lightweight and user-friendly e-mail client

Tutanota automatically encrypts all your data on your device. Your emails as well as your contacts stay private. You can easily communicate with any of your friends end-to-end encrypted. Even subject and attachments as well as all your contacts are encrypted. It provides an open webmail service that is very easy to use.

Tutanota - Encrypted MailBox

Mutt is a small but very powerful text-based mail client for Unix operating systems. Its features include message threading, POP3, IMAP support, support for multiple mailbox formats (mbox, MMDF, MH, maildir), Delivery Status Notification (DSN) support, multiple message tagging, translation into at least 20 languages and lot more.

Mutt - Text based E-Mail Client

SquirrelMail is a PHP-based Web email client. It includes built-in pure PHP support for IMAP and SMTP, and renders all pages in pure HTML 4.0 for maximum compatibility across browsers. It has strong MIME support and a flexible plugin system. 

SquirrelMail - Webmail

All your E-mail, from all your accounts, in one place. Cypht is not your father's webmail. Unless you are one of my daughters, in which case it is your father's webmail. Cypht is like a news reader, but for E-mail. Cypht does not replace your existing accounts - it combines them into one. And it's also a news reader..

Cypht - Lightweight Open Source webmail written in PHP and JavaScript

Geary is an email application built around conversations, for the GNOME 3 desktop. It allows you to read, find and send email with a straightforward, modern interface. 

Geary - Email Application to Send and Receive mails

ProtonMail provides secure email service. All emails are secured automatically with end-to-end encryption. This means even we cannot decrypt and read your emails. As a result, your encrypted emails cannot be shared with third parties.

ProtonMail - Web client for the ProtonMail secure email service

KMail is the email component of Kontact, the integrated personal information manager from KDE. It supports the standard mail protocols IMAP, POP3 and SMTP. It supports authentication via NTLM (Microsoft Windows) and GSSAPI (Kerberos). It has  native support for inline OpenPGP, PGP/MIME, and S/MIME.    

KMail - KDE Email Application

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