LMTP
Encyclopedia
The Local Mail Transfer Protocol (LMTP) is a derivative of ESMTP, the extension of the Simple Mail Transfer Protocol
. It is defined in RFC 2033.
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 storage server acting as a Mail Delivery Agent
. Mail queues are an inherent requisite of SMTP. In situations in which mail queues are not possible, LMTP is desirable, since a mail storage server should manage only its mail store without having to allocate more storage for a mail queue. This is not possible with SMTP when there are multiple recipients for a mail message. SMTP can only indicate successful delivery or failure for all or none of the recipients, creating the need for a separate queue to handle the failed recipients. LMTP, on the other hand, can indicate success or failure to the client for each recipient, allowing the client to handle the queueing instead. The client in this case would typically be an Internet-facing mail gateway. LMTP is not intended for use over wide area network
s. In other words, the MTA still handles all outgoing mail, including the mail stream from the LMTP agent, to another mail server located somewhere on the Internet.
LMTP is an Application Layer
protocol of the Internet Protocol Suite
. It uses a Transmission Control Protocol
(TCP) transport like SMTP, but must not use port number 25, the well-known port for SMTP.
LMTP conversation syntax is based on the same commands as Extended SMTP with the following exceptions:
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, LMTP is not run on the standard TCP port of SMTP.
Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...
. It is defined in RFC 2033.
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 storage server acting as a Mail Delivery Agent
Mail delivery agent
A mail delivery agent or message delivery agent is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox...
. Mail queues are an inherent requisite of SMTP. In situations in which mail queues are not possible, LMTP is desirable, since a mail storage server should manage only its mail store without having to allocate more storage for a mail queue. This is not possible with SMTP when there are multiple recipients for a mail message. SMTP can only indicate successful delivery or failure for all or none of the recipients, creating the need for a separate queue to handle the failed recipients. LMTP, on the other hand, can indicate success or failure to the client for each recipient, allowing the client to handle the queueing instead. The client in this case would typically be an Internet-facing mail gateway. LMTP is not intended for use over wide area network
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
s. In other words, the MTA still handles all outgoing mail, including the mail stream from the LMTP agent, to another mail server located somewhere on the Internet.
LMTP is an Application Layer
Application layer
The Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
protocol of the Internet Protocol Suite
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
. It uses a Transmission Control Protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) transport like SMTP, but must not use port number 25, the well-known port for SMTP.
LMTP conversation syntax is based on the same commands as Extended SMTP with the following exceptions:
- ESMTP's EHLO verb is replaced with LHLO.
- ESMTP requires a single status for the entire message body. LMTP requires a response for each previously successful RCPT command. That is, in case of multiple recipients, after the body of the message has been transmitted, LMTP can still fail for some recipient while succeeding for the others. With this facility, LMTP can fail if a user is over quota without the burden of generating Bounce messageBounce messageIn the Internet's standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt , a Delivery Status Notification message, a Non-Delivery Notification or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another...
s.
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, LMTP is not run on the standard TCP port of SMTP.