src/src/transports/README

   1 TRANSPORTS:
   2
   3 A delivery attempt results in one of the following values being placed in
   4 addr->transport_return:
   5
   6     OK          success
   7     DEFER       temporary failure
   8     FAIL        permanent failure
   9     PANIC       disaster - causes exim to bomb
  10
  11 The field is initialized to DEFER when the address is created, in order that
  12 unexpected process crashes or other problems don't cause the message to be
  13 deleted.
  14
  15 For non-OK values, additional information is placed in addr->errno,
  16 addr->more_errno, and optionally in addr->message. These are inspected only if
  17 the status is not OK, with one exception (see below).
  18
  19 In addition, the addr->special_action field can be set to request a non-default
  20 action. The default action after FAIL is to return to sender; the default
  21 action after DEFER is nothing. The alternatives are:
  22
  23   SPECIAL_NONE       (default) no special action
  24   SPECIAL_FREEZE     freeze the message
  25   SPECIAL_WARN       send warning message
  26
  27 The SPECIAL_WARN action is the exception referred to above. It is picked up
  28 only after a *successful* delivery; it causes a warning message to be sent
  29 containing the text of warn_message to warn_to. It can be used in appendfile,
  30 for example, to send a warning message when the mailbox size crosses a given
  31 threshold.
  32
  33 If the transport is handling a batch of several addresses, it may either put an
  34 individual value in each address structure, and return TRUE, or it may put a
  35 common value in the first address, and return FALSE.
  36
  37 Remote transports usually return TRUE and local transports usually return
  38 FALSE; however, the lmtp transport may return either value, depending on what
  39 happens inside it.
  40
  41 ****