Zimbra is the most capable open source email and collaboration suite available today. It provides support for Email, Shared calendar, Document management, Pop3, Address book and lot more. Mails could be tagged and search support is also provided. Zimbra is considered as an alternative to Microsoft Exchange Server.

Zimbra - Email and Collaboration Suite

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

James (a.k.a Java Apache Mail Enterprise Server) is a 100% pure Java SMTP and POP3 Mail server, and NNTP News server designed to be a complete and portable enterprise mail/messaging engine solution based on currently available open messaging protocols. 
 <ul><li>Is 100% pure java capable of running on Java 1.4 onwards</li>
<li>Integrates protocols
 <ul><li>Email
 <ul><li><a href="http://james.apache.org/server/rfclist/smtp/rfc0821.txt" class="externalLink"><abbr title="Simple Mail Transport Protocol">SMTP</abbr></a></li>
<li><a href="http://james.apache.org/server/rfclist/pop3/rfc1725.txt" class="externalLink"><abbr title="Post Office Protocol">POP3</abbr></a><ul><li>Outgoing server</li>
<li>Incoming from POP3 accounts (FetchMail)</li>
</ul>
</li>
<li><a href="http://james.apache.org/server/rfclist/imap4/rfc2060.txt" class="externalLink"><abbr title="Internet Message Access Protocol">IMAP</abbr></a> (James 3)
 <ul><li>Server</li>
<li><a href="http://james.apache.org/jsieve" class="externalLink">Sieve</a> 
 filtering into IMAP mailboxes for incoming mail</li>
</ul>
</li>
</ul>
</li>
</ul>
<ul><li><a href="http://james.apache.org/server/rfclist/nntp/rfc0977.txt" class="externalLink"><abbr title="Network News Transfer Protocol">NNTP</abbr></a> (better known as news)</li>
</ul>
</li>
<li>Is based on open standards</li>
<li>Is a mailet container. Processing is delegated to independent, extensible, pluggable agents
 specified by the <a href="http://james.apache.org/mailet/api" class="externalLink">Mailet API</a>. Any function
 which is not already available (from <a href="http://james.apache.org/mailet" class="externalLink">James</a>
 or from a third party) can be developed.</li>
<li>Is a modular, component based <a href="http://dbpedia.org/resource/Inversion_of_control" class="externalLink">IoC</a> 
 mail platform
 <ul><li>Based on mature, production proved <a href="http://excalibur.apache.org" class="externalLink">Apache Excalibur</a>
 components</li>
<li>James 2 (production) uses the <a href="http://avalon.apache.org" class="externalLink">Avalon</a> framework. Avalon development
 has now stopped but the framework is mature, stable and of proved production quality.</li>
<li><a href="http://james.apache.org/server/head" class="externalLink">James 3</a> (development) supports Spring and is moving
 towards OSGI</li>
</ul>
</li>
<li>Support to create mailing list </li>

</ul>

James (a.k.a Java Apache Mail Enterprise Server) is a 100% pure Java SMTP and POP3 Mail server, and NNTP News server designed to be a complete and portable enterprise mail/messaging engine solution based on currently available open messaging protocols.

James - Enterprise Mail Server

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

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

iRedMail is a fully fledged, full-featured mail server solution. It uses open source packages like Postfix, Dovecot, Apache, MySQL, OpenLDAP, Amavisd, RoundCube etc to provide this solution. 

IRedMail - Free, Open Source Email Server Solution for Linux

MailCore 2 provides a simple and asynchronous Objective-C API to work with the e-mail protocols IMAP, POP and SMTP. The API has been redesigned from the ground up.

MailCore 2 - Simple and asynchronous API to work with e-mail protocols IMAP, POP and SMTP

Dovecot is an open source IMAP and POP3 email server for Linux or UNIX-like systems written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It is fast, simple to set up, requires no special administration and it uses very little memory. Dovecot is NOT responsible for receiving mail from other servers. Dovecot presents mail already stored on the system to mail-user agents [MUA].

Dovecot - Secure IMAP and Pop3 server for Linux

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

WildDuck is a modern mail server software for IMAP and POP3. Modern being scalable, unicode-first and API-controlled. You can bundle WildDuck with Haraka and ZoneMTA to create a complete mail server. It uses a distributed database (sharded + replicated MongoDB) as a backend for storing all data, including emails. It tries to follow Gmail in product design. If there's a decision to be made then usually the answer is to do whatever Gmail has done.

WildDuck Mail Server -  Self hosted modern mail server `

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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