The LMTP protocol (documented in RFC2033) is a language similar to SMTP, but it is used to deliver messages to a program that does local, final delivery. LMTP uses an acknowledged protocol that allows each recipient's status to be reported individually, avoiding some of the problems of nonacknowledged delivery.

The F=z delivery agent flag causes the delivery agent to speak LMTP to the invoked delivery program. This delivery agent flag should be set only when an appropriate program is used. The easy way to use LMTP is described in the section dealing with the local_lmtp FEATURE (FEATURE(local_lmtp)). That feature uses the mail.local program that is supplied with the open source sendmail distribution. (See Section 5.3 for a full description of the mail.local program, and its various switches that can modify how it uses LMTP.)

Note that declaring the local_lmtp FEATURE causes this F=z delivery agent flag to be automatically included for the local delivery agent (as well as F=X, F=X).