Halon AlternativesMail Servers and other similar apps like Halon

Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail. It attempts to be fast, easy to administer, and secure. The software is also known by its former names VMailer and IBM Secure Mailer.

KumoMTA is the only true Open-Source PowerMTA alternative, capable of matching performance of several million messages per hour while providing granular traffic shaping configuration and automation across unlimited IPs.

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.

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.

MailerQ is a high performance Mail Transfer Agent (MTA) and a very fast SMTP server, designed to deliver large volumes of email messages at very high speeds. MailerQ is based on RabbitMQ AMQP.

EmailSuccess, an advanced SMTP Server (MTA) built from the ground up to handle the most critical reliability, performance, security, and deliverability requirements.