Local Mail Transfer Protocol
The Local Mail Transfer Protocol ( LMTP ) is a derivative of the SMTP protocol.
While SMTP is designed for the global mail delivery between any mail transfer agents, is LMTP for delivery of mail within a local organization, ie thought Agent for transport between different local server components such as Mail Transfer Agent and Mail Delivery. The delivery to the individual user mailboxes is done, however, by the Mail Delivery Agent.
The differences with SMTP
- Unlike SMTP, LMTP is designed so that it can be used for secure communication with servers that do not have a queue. If the delivery can not be made directly, the server the mail temporarily from. Only if the mail can be immediately fully delivered, the transfer via LMTP takes place.
- To avoid that a plain SMTP client communicates with a plain LMTP server LHLO is used instead of the SMTP HELO or EHLO typical handshake.
- If there were no successful RCPT command in the current session, the server MUST return the error code 503 response to DATA.
- After the final ". " the DATA command must send a status message per accepted RCPT command the server. The order of the client must be respected.
Example from the client's perspective
For better understanding, the following example is extended with comments that do not belong to the Protocol. These begin with # and extend to the end of the line.
LMTP is supported in several software packages, including