27. The lmtp transport

The lmtp transport runs the LMTP protocol (RFC 2033) over a pipe to a specified command or by interacting with a Unix domain socket. This transport is something of a cross between the pipe and smtp transports. Exim also has support for using LMTP over TCP/IP; this is implemented as an option for the smtp transport. Because LMTP is expected to be of minority interest, the default build-time configure in src/EDITME has it commented out. You need to ensure that

  TRANSPORT_LMTP=yes

is present in your Local/Makefile in order to have the lmtp transport included in the Exim binary.

The private options of the lmtp transport are as follows:

batch_id

See the description of local delivery batching in chapter 24.

batch_max

This limits the number of addresses that can be handled in a single delivery. Most LMTP servers can handle several addresses at once, so it is normally a good idea to increase this value. See the description of local delivery batching in chapter 24.

command

This option must be set if socket is not set. The string is a command which is run in a separate process. It is split up into a command name and list of arguments, each of which is separately expanded (so expansion cannot change the number of arguments). The command is run directly, not via a shell. The message is passed to the new process using the standard input and output to operate the LMTP protocol.

This option must be set if command is not set. The result of expansion must be the name of a Unix domain socket. The transport connects to the socket and delivers the message to it using the LMTP protocol.

timeout

The transport is aborted if the created process or Unix domain socket does not respond to LMTP commands or message input within this timeout.

Here is an example of a typical LMTP transport:

  lmtp:
    driver = lmtp
    command = /some/local/lmtp/delivery/program
    batch_max = 20
    user = exim

This delivers up to 20 addresses at a time, in a mixture of domains if necessary, running as the user exim.