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+

The JavaMail API is a set of abstract APIs that model a mail system. The API provides a platform independent and protocol independent framework to build Java technology based email client applications. The JavaMail API provides facilities for reading and sending email. Service providers implement particular protocols. Several service providers are included with the JavaMail API package; others are available separately. 
 
The JavaMail reference implementation jar file, includes SMTP, IMAP, and POP3 protocol providers.

The JavaMail API is a set of abstract APIs that model a mail system. The API provides a platform independent and protocol independent framework to build Java technology based email client applications. The JavaMail API provides facilities for reading and sending email. Service providers implement particular protocols. Several service providers are included with the JavaMail API package; others are available separately. 

JavaMail - Java library to build Mail and Messaging Applications

Mailu is a simple yet full-featured mail server as a set of Docker images. The project aims at providing people with an easily setup, easily maintained and full-featured mail server while not shipping proprietary software nor unrelated features often found in popular groupware.
 
Its features include: 
<ul>
<li>Standard email server, IMAP and IMAP+, SMTP and Submission</li>
<li>Advanced email features, aliases, domain aliases, custom routing</li>
<li>Web access, multiple Webmails and administration interface</li>
<li>User features, aliases, auto-reply, auto-forward, fetched accounts</li>
<li>Admin features, global admins, announcements, per-domain delegation, quotas</li>
<li>Security, enforced TLS, Letsencrypt!, outgoing DKIM, anti-virus scanner</li>
<li>Antispam, auto-learn, greylisting, DMARC and SPF</li>
<li>Freedom, all FOSS components, no tracker included</li>
</ul>

Mailu is a simple yet full-featured mail server as a set of Docker images. The project aims at providing people with an easily setup, easily maintained and full-featured mail server while not shipping proprietary software nor unrelated features often found in popular groupware.

Mailu - Mail server as a set of Docker images

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

Postfix is mail server that started life at IBM research as an alternative to the widely-used Sendmail program. Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different. It supports IPv6, DKIM, DomainKeys and SenderID authentication, SPF, Sendmail Milter (mail filter) protocol, Zombie blocker, TLS encryption, LDAP and it supports database like MySQL, PostgreSQL, Berkeley DB, Memcache.

Postfix - Mail Server

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

The deepOfix Mail Server makes it possible for organisations with minimal technical expertise to have robust messaging infrastructure. It has built in LDAP support, SMTP, POP3 and IMAP services.

deepOfix - Mail Server

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

Qpopper is the most widely-used server for the POP3 protocol (this allows users to access their mail using any POP3 client). Qpopper is normally used with standard UNIX mail transfer and delivery agents such as sendmail or smail. It supports IPV6, TLS / SSL and lot more.

Qpopper - The most widely-used POP3 server for Linux

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

Fetchmail is a full-featured, robust, well-documented remote-mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections). It supports every remote-mail protocol now in use on the Internet: POP2, POP3, RPOP, APOP, KPOP, all flavors of IMAP, ETRN, and ODMR. It can even support IPv6 and IPSEC. 
 
Fetchmail retrieves mail from remote mail servers and forwards it via SMTP, so it can then be read by normal mail user agents such as mutt, elm(1) or BSD Mail. It allows all your system MTA's filtering, forwarding, and aliasing facilities to work just as they would on normal mail. Fetchmail can be used as a POP/IMAP-to-SMTP gateway for an entire DNS domain, collecting mail from a single drop box on an ISP and SMTP-forwarding it based on header addresses. (We don't really recommend this, though, as it may lose important envelope-header information. ETRN or a UUCP connection is better.)

Fetchmail is a full-featured, robust, well-documented remote-mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections). It supports every remote-mail protocol now in use on the Internet: POP2, POP3, RPOP, APOP, KPOP, all flavors of IMAP, ETRN, and ODMR. It can even support IPv6 and IPSEC.

fetchmail - TLS-enabled mail retrieval and forwarding agent

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