Postfix!

Sendmail is a general purpose internetwork email routing facility that supports many kinds of transfer and delivery methods, including the Simple Mail Transfer Protocol (SMTP) used for email transport over the Internet. A descendant of the delivermail program written by Eric Allman, Sendmail is a well-known project of the free and open source software and Unix communities, and has spread both as free software and proprietary software.

Haraka is an open source SMTP server written in Node.js which provides extremely high performance coupled with a flexible plugin system allowing Javascript programmers full access to change the behaviour of the server. It is used heavily in some high traffic sites - see the "Known Users" link above for testimonials.

The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be customized, enabled or disabled at will.

MailerQ is a high performance Mail Transfer Agent (MTA) and a very fast SMTP server, designed for delivering large volume email messages at very high speeds. MailerQ is fast. Its sole purpose is to send emails. Storing, queuing and prioritizing messages is done in a separate AMQP message broker RabbitMQ MailerQ gives full control over queued messages. Monitor email delivery and intervene when necessary. Prioriziting and routing messages has never been easier..

Available in Windows and Windows Server via the optional IIS role.

PowerMTA is an “enterprise” grade infrastructure application for email built to address the challenges of critical digital messaging environments and customer communications. PowerMTA™ is (advanced smtp software) providing unsurpassed reliability and delivery execution, with granular connection controls. Offering superior message throughput, simplified set up and configuration, and visibility through real-time reporting and tracking, PowerMTA™ pays dividends immediately. It is built on advanced multi-threaded persistent architecture—for messaging breakthrough that delivers unsurpassed resilience and performance. »

OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with some additional standard extensions. It allows ordinary machines to exchange e-mails with other systems speaking the SMTP protocol.

In the default mode, it transmits a mail to an SMTP server (for example at a free mail provider) which takes care of further delivery.To use this program with your mail user agent (MUA), create a configuration file with your mail account(s) and tell your MUA to call msmtp instead of /usr/sbin/sendmail.

A small Mail Transport Agent (MTA), designed for home and office use. Part of DragonFly BSD , it accepts mails from locally installed Mail User Agents (MUA) and delivers the mails either locally or to a remote destination. Remote delivery includes several features like TLS/SSL support and SMTP authentication. dma is not intended as a replacement for real, big MTAs like Sendmail or Postfix. Consequently, dma does not listen on port 25 for incoming connections. »

SSMTP is a program which delivers email from a local computer to a configured mailhost (mailhub). It is not a mail server (like feature-rich mail server sendmail) and does not receive mail, expand aliases or manage a queue. One of its primary uses is for forwarding automated email (like system alerts) off your machine and to an external email address.

An SMTP client that is extremely easy to configure and use, providing a simple HTTP API that is used for sending emails.

EmailSuccess is an advanced SMTP Server (MTA) derived from the technology and the expertise accumulated in years of exchanges with the ISP and ESP worlds, built from the ground up to handle the most critical reliability, performance, security, and deliverability requirements.

BulkMTA is Commercial Mail Server Similar to PowerMTA.IP Rotation, DKIM, Sender / Receiver Rate limiting