Open source email relay server

Here are some open-source email relay servers:

1. Postfix: A popular and widely-used open-source email relay server. It's highly configurable and supports various authentication methods.
2. Sendmail: Another well-known open-source email relay server that's been around for decades. It's known for its flexibility and customization options.
3. OpenSMTPD: A modern, open-source email relay server that's designed to be secure and easy to use. It's written in C and supports various authentication methods.
4. Exim: A widely-used open-source email relay server that's known for its reliability and performance. It's highly customizable and supports various authentication methods.
5. Qmail: A lightweight, open-source email relay server that's designed to be secure and easy to use. It's known for its simplicity and ease of configuration.
6. Milter: A open-source email relay server that's designed to be highly customizable and flexible. It supports various authentication methods and is known for its reliability.
7. Mailhog: A open-source email relay server that's designed to be easy to use and highly customizable. It supports various authentication methods and is known for its reliability.
8. Mailserver: A open-source email relay server that's designed to be highly customizable and flexible. It supports various authentication methods and is known for its reliability.

Some of the key features of these open-source email relay servers include:

• Support for various authentication methods (e.g. SMTP-AUTH, SPF, DKIM)
• Support for various email protocols (e.g. SMTP, POP3, IMAP)
• Highly customizable and configurable
• Support for spam filtering and virus scanning
• Support for multiple domains and users
• Highly scalable and reliable

When choosing an open-source email relay server, consider the following factors:

• Ease of installation and configuration
• Customizability and flexibility
• Support for various authentication methods and email protocols
• Reliability and scalability
• Security features (e.g. spam filtering, virus scanning)
• Community support and documentation

It's also important to note that while open-source email relay servers can be a cost-effective and customizable solution, they may require more technical expertise and maintenance than commercial solutions.