Exim
Exim is a mail transfer agent (MTA) used in Unixllike operating systems. The first version was written in 1995 by Philip Hazel for use in the University of Cambridge Computing Service's e-mail systems. Exim is distributed under the GPL, and therefore is free to download, use and modify. Exim somewhat resembles Smail 3, but it has diverged and now surpasses it in user friendliness and flexibility. They both follow the Sendmail design model where a single main binary controls all the facilities of the MTA. This monolithic design is considered by some to be inherently less secure and slower, but despite this, Exim's security record is much better than Sendmail and comparable with Qmail and Postfix, as is its speed. In advanced areas such as queue handling, address routing and testing, it exhibits excellent performance. Exim doesn't have a Graphical User Interface (GUI) to help you configure it, but some Linux distributions add one.
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Haraka
Haraka can serve thousands of concurrent connections and deliver thousands of messages per second. Haraka and plugins are written in asynchronous JS and are very fast. Haraka has very good spam protection (see plugins) and works well as a filtering MTA. It also works well as an MSA running on port 587 with auth and dkim_sign plugins enabled. Haraka makes no attempt to be a mail store (like an exchange or postfix/exim/qmail), an LDA, or an IMAP server (like Dovecot or courier). Haraka is typically used with such systems. Haraka has a scalable outbound mail delivery engine built in. Mail marked as relaying (such as via an auth plugin) is automatically queued for outbound delivery. Haraka's plugin architecture provides an easily extensible MTA that complements traditional MTAs that excel at managing mail stores but do not have sufficient filtering.
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Mailu
Mailu is a simple yet full-featured mail server as a set of Docker images. It is free software (both as in free beer & as in free speech), open to suggestions and external contributions. 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.
Main features include:
Standard email server, IMAP and IMAP+, SMTP and Submission with autoconfiguration profiles for clients
Advanced email features, aliases, domain aliases, custom routing
Web access, multiple Webmails and administration interface
User features, aliases, auto-reply, auto-forward, fetched accounts
Admin features, global admins, announcements, per-domain delegation, quotas
Security, enforced TLS, DANE, MTA-STS, Letsencrypt!, outgoing DKIM, anti-virus scanner, Snuffleupagus, block malicious attachments
Antispam, auto-learn, greylisting, DMARC& SPF, anti-spoofing
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Mercury Mail Transport System
Mail from the outside world is received by Mercury and placed in the addressee's mailbox, where the user can access it at any later point. Messages sent by local users to the outside world are passed to Mercury, which then takes whatever steps are necessary to deliver them, removing the burden from the user's workstation and allowing him to continue with other work. If you connect using a dialup connection, then only the mail server needs to be able to access that connection. Your workstations do not need their own modems or Internet accounts. The mail server can continue processing mail even when the client workstations are turned off, allowing functions that depend on a continuously available service, such as automatic replies and auto-forwarding.
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