The Local Mail Transfer Protocol (LMTP) is an alternative to (Extended) Simple Mail Transfer Protocol for situations where the receiving side does not have a mail queue, such as a message transfer agent acting as a message delivery agent.
[1] Mail queues are an inherent requisite of SMTP.
The client in this case would typically be an Internet-facing mail gateway.
[2] LMTP conversation syntax is based on the same commands as SMTP (formerly called Extended SMTP) with the following exceptions:[1] The key difference is that LMTP will reject a message for a specific final recipient if it is not immediately deliverable.
For this reason, LMTP is not run on the standard TCP port of SMTP.