Exim
An SMTP MTA with programmable ACLs, routers, and transports. Plan message policy, delivery targets, and queue operations separately from mailbox access.
Start with the first guide, then choose a sending, receiving, or feedback task. Each walkthrough identifies permissions, operational checks, and the boundary between the service and MailKite. Refer to the official Exim documentation for current platform requirements.
Explore the guides
Install Exim4 on an Ubuntu test server
Install Exim4, choose local-only operation, and understand Ubuntu's generated configuration before opening SMTP to a domain.
Prepare Exim host identity and SMTP acceptance policy
Choose the packaged configuration model and validate local domains, relay permissions, and receiving ACLs before exposing SMTP.
Trace Exim routing and delivery transports
Use address routing tests to understand which router and transport handle a recipient before changing live delivery rules.
Diagnose Exim queued and frozen messages
Inspect queue summaries, message logs, and retry causes without deleting mail or forcing every message through a broken route.