E-MailRelay is described as 'A lightweight mail server with SMTP and POP3 support. It mainly used as store-and-forward mail server with POP access to spooled messages or it can be used as a personal internet mail server' and is a Mail Server in the office & productivity category. There are more than 10 alternatives to E-MailRelay for a variety of platforms, including Linux, Mac, Self-Hosted, BSD and Windows apps. The best E-MailRelay alternative is Postfix, which is both free and Open Source. Other great apps like E-MailRelay are Mail-in-a-box, Haraka, Exim and OpenSMTPD.
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.
Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
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.
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.
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.
Configure SMTP e-mail in IIS when you want to deliver e-mail messages from your site. Mail can be delivered immediately or it can be delivered to a file location on disk where it can be retrieved for delivery later.
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.
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The Postfix is more popular smtp server but it's more complicated to configure and it's Lunux/Unix only while the ER works on Windows too.