TRANSPORTS:

A delivery attempt results in one of the following values being placed in
addr->transport_return:

    OK          success
    DEFER       temporary failure
    FAIL        permanent failure
    PANIC       disaster - causes exim to bomb

The field is initialized to DEFER when the address is created, in order that
unexpected process crashes or other problems don't cause the message to be
deleted.

For non-OK values, additional information is placed in addr->errno,
addr->more_errno, and optionally in addr->message. These are inspected only if
the status is not OK, with one exception (see below).

In addition, the addr->special_action field can be set to request a non-default
action. The default action after FAIL is to return to sender; the default
action after DEFER is nothing. The alternatives are:

  SPECIAL_NONE       (default) no special action
  SPECIAL_FREEZE     freeze the message
  SPECIAL_WARN       send warning message

The SPECIAL_WARN action is the exception referred to above. It is picked up
only after a *successful* delivery; it causes a warning message to be sent
containing the text of warn_message to warn_to. It can be used in appendfile,
for example, to send a warning message when the mailbox size crosses a given
threshold.

If the transport is handling a batch of several addresses, it may either put an
individual value in each address structure, and return TRUE, or it may put a
common value in the first address, and return FALSE.

Remote transports usually return TRUE and local transports usually return
FALSE; however, the lmtp transport may return either value, depending on what
happens inside it.

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