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The only remote transport (smtp
) is normally configured to handle more than
one address at a time, so that when several addresses are routed to the same
remote host, just one copy of the message is sent. Local transports, however,
normally handle one address at a time. That is, a separate instance of the
transport is run for each address that is routed to the transport. A separate
copy of the message is delivered each time.
In special cases, it may be desirable to handle several addresses at once in a local transport, for example:
appendfile
transport, when storing messages in files for later
delivery by some other means, a single copy of the message with multiple
recipients saves space.
lmtp
transport, when delivering over "local SMTP" to some process,
a single copy saves time, and is the normal way LMTP is expected to work.
pipe
transport, when passing the message
to a scanner program or
to some other delivery mechanism such as UUCP, multiple recipients may be
acceptable.
These three local transports all have the same options for controlling multiple
("batched") deliveries, namely batch_max
and batch_id
. To save
repeating the information for each transport, these options are described here.
The batch_max
option specifies the maximum number of addresses that can be
delivered together in a single run of the transport. Its default value is one
(no batching). When more than one address is routed to a transport that has a
batch_max
value greater than one, the addresses are delivered in a batch
(that is, in a single run of the transport with multiple recipients), subject
to certain conditions:
$local_part
, no
batching is possible.
$domain
, only
addresses with the same domain are batched.
batch_id
is set, it is expanded for each address, and only those
addresses with the same expanded value are batched. This allows you to specify
customized batching conditions. Failure of the expansion for any reason,
including forced failure, disables batching, but it does not stop the delivery
from taking place.
In the case of the appendfile
and pipe
transports, batching applies
both when the file or pipe command is specified in the transport, and when it
is specified by a redirect
router, but all the batched addresses must of
course be routed to the same file or pipe command. These two transports have an
option called use_bsmtp
, which causes them to deliver the message in
"batched SMTP" format, with the envelope represented as SMTP commands. The
check_string
and escape_string
options are forced to the values
check_string = "." escape_string = ".." |
when batched SMTP is in use. A full description of the batch SMTP mechanism is
given in section Outgoing batched SMTP. The lmtp
transport does not have a
use_bsmtp
option, because it always delivers using the SMTP protocol.
If the generic envelope_to_add
option is set for a batching transport, the
Envelope-to: header that is added to the message contains all the addresses
that are being processed together. If you are using a batching appendfile
transport without use_bsmtp
, the only way to preserve the recipient
addresses is to set the envelope_to_add
option.
If you are using a pipe
transport without BSMTP, and setting the
transport's command
option, you can include $pipe_addresses
as part of
the command. This is not a true variable; it is a bit of magic that causes each
of the recipient addresses to be inserted into the command as a separate
argument. This provides a way of accessing all the addresses that are being
delivered in the batch. Note: This is not possible for pipe commands that
are specififed by a redirect
router.
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