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

Roundcube Webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an email client, including MIME support, address book, folder management, message searching and spell checking. 

Roundcube Webmail - Browser-based multilingual IMAP client

PHPMailer is a full-featured email creation and transfer class for PHP. Many PHP developers need to send email from their code. The only PHP function that supports this directly is mail(). However, it does not provide any assistance for making use of popular features such as encryption, authentication, HTML messages, and attachments. PHPMailer's integrated SMTP client allows email sending on all platforms without needing a local mail server. 

PHPMailer – A full-featured email creation and transfer class for PHP

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

SharpWebMail is an ASP.NET webmail application that is written in C#. It uses a POP3 or IMAP servers as the mailstore and sends mail through a SMTP server. It is very simple to configure (only a few settings in the web.config file). You can compose HTML messages, search your inbox, read complex mime messages, have multiple address books and much more.

SharpWebMail - ASP.NET Webmail Application

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

Swift Mailer is a component based mailing solution for PHP. It is highly object-oriented by design and lends itself to use in complex web application with a great deal of flexibility.

Swift Mailer - Comprehensive mailing tools for PHP

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

Mailpile is a modern web-mail you run on your own computer. It is a personal e-mail searching and indexing tool, largely inspired by Google's popular proprietary-but-gratis e-mail service. It wants to eventually become a fast and flexible back-end for awesome personal mail clients, including webmail. It supports importing archived mails from Gmail, IMAP server, Mbox etc. It supports PGP encryption and verification of emails and recipients. 

Mailpile - Mordern Web Mail

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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