TRANSPORTS:

A delivery attempt results in a status code being placed in addr->errno,
additional error information (optionally) being placed in addr->more_errno, and
an optional message in addr->message. The yield of the function is placed in
addr->transport_return, and is one of:

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

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

The errno, message, and special_action fields are looked at by the caller only
if the returned value is not OK. The returned value 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.

One exception to the above is the special_action value

  SPECIAL_WARN       send warning message

This 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 warnign message when the mailbox size
crosses a given threshold.

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