OpenSMTPD
The OpenBSD project's SMTP daemon, also available in portable builds. Match rules choose delivery actions; mailbox access and application processing are separate components.
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 OpenSMTPD documentation for current platform requirements.
Explore the guides
Install OpenSMTPD on a Debian Linux test server
Install Debian's OpenSMTPD package, create a local-only Maildir configuration, and validate it before configuring public SMTP.
Configure OpenSMTPD match rules and delivery actions
Choose a supported portable/OpenBSD build, define destination tables, and validate ordered match rules before accepting SMTP.
Secure OpenSMTPD submission and upstream relay access
Separate authenticated application listeners from Internet MX traffic and protect smarthost credentials and TLS configuration.
Inspect OpenSMTPD queue envelopes and delivery failures
Read queued envelope state, correlate logs, and choose a targeted recovery action without discarding outstanding messages.