LMTP

LMTP, or Local Mail Transfer Protocol, is a simple extension of SMTP designed for delivering mail to a local delivery agent or mailbox store. It is intended for communications between a mail transfer agent (MTA) and a local delivery agent or mail store, rather than for remote Internet transport. A key feature of LMTP is per-recipient delivery status reporting, allowing the MTA to attempt delivery to multiple recipients in a single session while reporting success or failure for each recipient individually.

Operation and differences from SMTP: An LMTP session begins with a handshake using LHLO, followed by MAIL FROM and RCPT TO commands for each recipient. After each RCPT TO, the server provides a per-recipient status, enabling selective acceptance or rejection of recipients within the same message. Once all recipients have been specified, the client sends DATA to transmit the message content. The server completes the transaction by returning final per-recipient dispositions and closing the session. Unlike standard SMTP, LMTP emphasizes per-recipient outcomes rather than a single overall status for the entire message.

Use and implementations: LMTP is commonly used for delivery from MTAs to local delivery systems or mailbox stores. It is implemented in several widely used MTAs and delivery agents, such as Postfix, Exim, and Dovecot’s LMTP service, as well as Cyrus IMAP. Deployments typically run LMTP over TLS and may employ authentication depending on the environment and security requirements, though it is primarily intended for local delivery rather than remote transfer.

Standardization and scope: LMTP was defined in RFC 2033 (Local Mail Transfer Protocol) and has since been implemented and extended in various systems. While suitable for efficient local delivery with per-recipient reporting, LMTP is not generally used for Internet-wide transport; SMTP remains the standard for remote delivery.