smtpd

smtpd is the network-facing daemon that implements the Simple Mail Transfer Protocol (SMTP) for receiving mail from other servers. It listens on common SMTP ports (typically 25 for general mail and 587 for submission), negotiates SMTP sessions, validates sender and recipient addresses, and passes accepted messages to the local delivery system or forwards them to other MTAs.

In a typical mail transfer agent architecture, smtpd accepts mail from remote servers, enforces basic policy during the SMTP session, and then forwards the message to a local delivery mechanism via LMTP or SMTP to a local MTA instance. It may perform checks such as relaying restrictions, basic anti-spam measures, and initial address verification before handing off mail to the local delivery subsystem.

Many MTAs implement smtpd as a separate, dedicated process. Features commonly associated with smtpd include support for TLS (often via STARTTLS), authentication for submission clients (SASL), access control lists, and rate limiting. It may also integrate with content filtering, anti-spam tools, and DNS-based checks to assess or block incoming connections.

Configuration and behavior of smtpd vary by MTA. In Postfix, for example, smtpd is configured within the master process and main configuration files; other MTAs such as OpenBSD’s smtpd provide their own configuration language and policy framework. Security considerations include avoiding open relays, enabling encryption and authentication for submission, and running smtpd with least privilege and appropriate confinement.

Overall, smtpd serves as the entry point for inbound mail in many mail systems, mediating connections, enforcing basic policies, and delegating message delivery to the local mail infrastructure.