LMTP (Local Mail Transfer Protocol) is an email transfer protocol which is designed as an alternative to Extended SMTP (ESMTP). It was first defined in RFC 2033 and Local Mail Transfer Protocol inherits most of its features and usages from ESMTP, with a few modifications. Like SMTP, Local Mail Transfer Protocol is an Application Layer protocol that relies on TCP transport.
- LMTP uses the syntax and semantics of ESMTP’s commands.
- LHLO replaces ESMTP`s standard greeting, EHLO.
- LMTP rejects the messages that are not immediately deliverable to its destination, thus not requiring a queue. Hence Local Mail Transfer Protocol never runs on TCP port 25, the default SMTP port.
- LMTP requires a response for each previously successful RCPT command (not simply for the entire message body, like ESMPT). This makes it possible for Local Mail Transfer Protocol to succeed for some recipients and failure for others.
When a mail storage space for a queue cannot be allocated by the mail storage server or an MDA and also prevents the use of conventional SMTP (which can only report delivery success or failure for all or none of the recipients and requires a separate queue for the failed recipients), then Local Mail Transfer Protocol can be called. Unlike ESMP, Local Mail Transfer Protocol is capable of informing the client of delivery success or failure per each recipient, thus permitting the client (usually an Internet-facing gateway) to do the queuing.
Restrictions on the use of LMTP
- As its name suggests, Local Mail Transfer Protocol is used locally, not over WANs (wide area networks).
- The use of Local Mail Transfer Protocol must be specifically prearranged between the parties involved.
- Local Mail Transfer Protocol may not be used on TCP port 25 as it is reserved for SMTP.