ExIm

Exim is a mail transfer agent (MTA) for Unix-like operating systems. It routes, delivers, and receives email using the Simple Mail Transfer Protocol (SMTP) and related standards. Exim is designed to be highly configurable and secure, with a flexible configuration language that allows administrators to define how mail should be routed, delivered, and filtered through a system of routers and transports.

Originating in the mid-1990s at the University of Cambridge, Exim was developed as an alternative to other MTAs and has been distributed as free software under the Exim license. It has since become widely used across a range of Unix-like systems and Linux distributions.

Architecture and features include a daemon model where a master process manages worker processes for delivery and queue handling. The configuration describes routers (decision points for routing mail) and transports (delivery methods), along with ACLs (access control lists) that enforce policy. Exim supports IPv4 and IPv6, encryption via TLS (including STARTTLS), and authentication via SASL. It can act as both an SMTP client and an SMTP server, delivering mail to local mailboxes or remote destinations, and it provides extensive logging, queue management, and error reporting. The MTA also supports DNS-based lookups, configurable spam filtering interfaces, and the integration of external content filters and antivirus tools.

Deployment and ecosystem: Exim is widely deployed on Linux and BSD systems, and it is known for its comprehensive documentation and a large ecosystem of patches, tutorials, and integration options. It emphasizes flexibility and reliability in complex mail-processing environments and remains active with ongoing maintenance and feature updates.