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

•    C

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.

Sylpheed - Lightweight and user-friendly e-mail client

•    GTK

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.

Mailspring - A beautiful, fast Webmail

•    Javascript

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.

K-9 Mail – Advanced Email for Android

•    Java

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

Thunderbird - Email Client

•    C++

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.

Mailpile - A free & open modern, fast email client with user-friendly encryption and privacy features

•    Python

Mailpile (https://www.mailpile.is/) is a modern, fast web-mail client with user-friendly encryption and privacy features. The development of Mailpile is funded by a large community of backers and all code related to the project is and will be released under an OSI approved Free Software license. Mailpile places great emphasis on providing a clean, elegant user interface and pleasant user experience. In particular, Mailpile aims to make it easy and convenient to receive and send PGP encrypted or signed e-mail.

KMail - KDE Email Application

•    C++

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.

Cypht - Lightweight Open Source webmail written in PHP and JavaScript

•    PHP

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..

Citadel

•    C

Citadel is a collaboration suite (messaging and groupware). It provides support for Email, Calendaring/Scheduling, Address books, Bulletin boards, Mailing List Server, Instant Messaging, Wiki, Multiple domain support.

SquirrelMail - Webmail

•    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.

Rainloop Webmail - Simple, modern & fast web-based email client

•    PHP

Simple, modern & fast web-based email client.Modest system requirements, decent performance, simple installation and upgrade, no database required - all these make RainLoop Webmail a perfect choice for your email solution.

Geary - Email Application to Send and Receive mails

•    C++

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.

Mail Test Agent (MTA)

•    CSharp

Project to test email setup & connectivity. Includes modules for sending email (SSL Support), ping, port scan, & dns lookup (A, CNAME, & MX records).

Notmuch -- Just an email system

•    C

Notmuchmail helps to do fast, global-search and tag-based email system to use within your text editor or in a terminal. "Not much mail" is what Notmuch thinks about your email collection. Even if you receive 12000 messages per month or have on the order of millions of messages that you've been saving for decades. Regardless, Notmuch will be able to quickly search all of it. It's just plain not much mail.

Enkive - Email Archiving and eDiscovery software

•    Java

Enkive is a e-mail archiving and retrieval application. It captures e-mail messages as they arrive or are sent to ensure they are retained before a worker can delete them in an e-mail client. The archived mails could be searched and retrieved. Enkive can import mail from a file in mbox format. It dedupicates email and attachments, only one copy of the messages are stored.

e-volve mail client

•    Java

An email client which basically focuses on sorting incoming emails for better email management using classification algorithms. Implemented for windows platform. The e-mail client focuses on inbox management, categorization of e-mails usig algorithms

Mutt - Text based E-Mail Client

•    C

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.

node-imap - An IMAP client module for node.js.

•    Javascript

node-imap is an IMAP client module for node.js.This module does not perform any magic such as auto-decoding of messages/attachments or parsing of email addresses (node-imap leaves all mail header values as-is).

Brahmi Mail

Brahmi Email Client is a Java based email client that aims to support features of full fledged email client. It also budles with it Java based Brahmi Input Methods available from http://brahmi.sourceforge.net

Hastymail - IMAP/SMTP client in PHP

•    PHP

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.