Which email protocol's name is commonly shortened to LMTP?

Which email protocol’s name is commonly shortened to LMTP? LMTP stands for Local Mail Transfer Protocol.

LMTP is an alternative to SMTP intended mainly for local delivery, especially when the receiving system does not maintain its own mail queue. A mail-transfer agent can hand messages to a delivery service that stores them directly in users’ mailboxes, leaving queue management to the upstream system.

Its command syntax resembles SMTP, but LMTP uses LHLO instead of HELO or EHLO. Its most important operational difference is per-recipient reporting: after the message body is transmitted, the server can accept delivery for some recipients and reject it for others. That is useful when one mailbox is over quota while another is available.

LMTP is not meant for ordinary Internet-wide relay and must not use SMTP’s standard TCP port 25. Port 24 has been used by some implementations, although local Unix-domain sockets are also common. The word “local” describes its intended delivery environment, not a different expansion such as “Local Message Tracking Protocol.”