Postfix: The Definitive Guide

Local Mail Transfer Protocol

Some POP/IMAP servers use nonstandard message stores. Since it would be unreasonable to expect MTAs such as Postfix to understand many different proprietary formats, the Local Mail Transfer Protocol (LMTP) provides a way to pass email messages from one local mail service to another without depending on a common message store. LMTP is based on, and is a simplified version of, SMTP. With LMTP, the server can either accept an email message immediately or it cannot accept it at all. There is no attempt by the LMTP server to queue or redeliver a message that cannot be delivered immediately.

When an MTA makes a delivery to an SMTP server, where the message is destined for multiple recipients, and one or more recipients cannot accept the message for some reason, the SMTP server takes the responsibility of queuing the message to deliver it later, and reports an overall successful delivery to the MTA. LMTP servers do not queue messages, so they must return an individual status reply for every recipient of a particular email message. For those recipients that could not be delivered, the MTA, and not the LMTP server, takes the responsibility of queuing the message and attempting redelivery.

LMTP conversations can occur between mail subsystems on the same machine or on different machines on a local area network. It is not recommended for wide area networks, since the protocol depends on a quick response to indicate whether the message was delivered. With SMTP there ...