LMTP

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The Local Mail Transfer Protocol or LMTP is a derivative of SMTP, the Simple Mail Transfer Protocol. LMTP is designed as an alternative to normal SMTP for situations where the receiving side does not have a mail queue, such as a Mail Delivery Agent that understands SMTP conversations.

LMTP is an application layer protocol, which runs on top of TCP/IP.

An LMTP conversation uses the same commands as an ESMTP conversation with the following exceptions:

ESMTP's EHLO verb is replaced with LHLO
ESMTP requires a single status for the entire message from the server after the client sends the message's DATA. LMTP requires a response for each previously accepted RCPT command.

The key difference is that LMTP will reject a message if it is not immediately deliverable to its final destination. This removes the need for a mail queue. For this reason, one is not supposed to run an LMTP server on the TCP/25 port.