- LMTP
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 storage server acting as a "Mail Delivery Agent ". This 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 overwide area network s.LMTP is an
application layer protocol, which runs on top ofTCP/IP .An LMTP conversation uses the same commands as an ESMTP conversation with the following exceptions:
* ESMTP's EHLO verb is replaced with LHLO.
* LMTP should not listen on port 25.
* 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 and succeed for the others. That way, LMTP can fail if a user is over quota without the burden of generatingBounce message 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, one is not supposed to run an LMTP server on the TCP/25 port.
Related RFC
* RFC 2033 — The Local Mail Transfer Protocol
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