Exim

Exim is a message transfer agent (MTA) for Unix-like systems, originally developed at the University of Cambridge. It handles SMTP email transport and is designed for highly flexible routing and policy-driven control over how mail is accepted, processed, and delivered.

Key Features

• SMTP server and outgoing transport for Internet email delivery
• Highly configurable routing and delivery logic for complex mail flows
• Extensive access control and checks for incoming mail during SMTP transactions
• Can replace sendmail-style MTAs, with a distinct configuration model
• Supports integrations commonly used in mail stacks (for example TLS and SMTP authentication)

Use Cases

• Operating a mail server that accepts and relays inbound/outbound SMTP mail
• Building policy-heavy mail routing for multi-domain or multi-tenant environments
• Providing a configurable MTA component alongside IMAP/POP3 and spam-filtering systems

Limitations and Considerations

• Configuration is powerful but can be complex compared to simpler MTAs
• Security fixes are important; older releases can be obsolete and should be avoided

Exim remains a widely used, flexible MTA for administrators who need fine-grained control over mail routing and inbound SMTP policy. When maintained and kept up to date, it can serve as the core SMTP transport component in a full email infrastructure.