[Note: this document still needs more examples]

Postfix VERP support
====================

Postfix supports variable envelope return path addresses on request.
When VERP style delivery is requested, each recipient of a message
receives a customized copy of the message, with his/her own recipient
address encoded in the envelope sender address.

For example, when VERP style delivery is requested, Postfix delivers
mail from owner-listname@origin for a recipient user@domain, with
a sender address that encodes the recipient as follows:

    owner-listname+user=domain@origin

Thus, undeliverable mail can reveal the undeliverable recipient
address without requiring the list owner to parse bounce messages.

The VERP concept was popularized by the qmail MTA and by the ezmlm
mailing list manager.

The whole process is controlled by four configuration parameters.

- default_verp_delimiters (default value:  +=) controls what VERP
delimiter characters Postfix uses when VERP style delivery is
requested but no explicit delimiters are specified.

- verp_delimiter_filter (default: -+=) controls what characters
Postfix accepts as VERP delimiter characters on the sendmail command
line and in SMTP commands. Many characters must not be used as VERP
delimiter characters, either because they already have a special
meaning in email addresses (such as the @ or the %), because they
are used as part of a username or domain name (such as alphanumerics),
or because they are non-ASCII or control characters.  And who knows,
some characters may tickle bugs in vulnerable software.

- authorized_verp_clients (default value: $mynetworks) controls
what network clients are allowed to request VERP style delivery.
Exceptions:  the Postfix QMQP server uses its own access control
mechanism, and local submission (via /usr/sbin/sendmail etc.) is
always authorized. To authorize a host, list its name, IP address,
subnet (net/mask) or parent .domain.

- disable_verp_bounces (default: no) controls if Postfix sends one
bounce report for multi-recipient VERP mail, or one bounce report
per recipient. The default, one per recipient, is what ezmlm needs.

Using VERP with majordomo etc. mailing lists
============================================

In order to make VERP useful with majordomo etc. mailing lists,
you would configure the list manager to submit mail according
to one of the following two forms:

    sendmail -V -f owner-listname other-arguments...

    sendmail -V+= -f owner-listname other-arguments...

The first form uses the default main.cf VERP delimiter characters.
The second form allows you to explicitly specify the VERP delimiter
characters. The example shows the recommended values.

This text assumes that you have set up an owner-listname alias that
routes undeliverable mail to a real person:

    /etc/aliases:
	owner-listname: yourname+listname

In order to process bounces we are going to make extensive use of
address extension tricks.

You need to tell Postfix that + is the separator between an address
and its optional address extension, that address extensions are
appended to .forward file names, and that address extensions are
to be discarded when doing alias expansions:

    /etc/postfix/main.cf:
	recipient_delimiter = +
	forward_path = $home/.forward${recipient_delimiter}${extension},$home/.forward
	propagate_unmatched_extensions = canonical, virtual

(the last two parameter settings are default settings).

You need to set up a file named .forward+listname with the commands
that process all the mail that is sent to the owner-listname address:

   ~/.forward+listname:
	"|/some/where/command ..."

With this set up, undeliverable mail for user@domain will be returned
to the following address:

    owner-listname+user=domain@your.domain

which is processed by the command in your .forward+listname file.
The message should contain, among others, a To: header with the
encapsulated recipient sender address:

    To: owner-listname+user=domain@your.domain

It is left as an exercise for the reader to parse the To: header
line and to pull out the user=domain part from the recipient address.

VERP support in the Postfix SMTP server
=======================================

The Postfix SMTP server has a new command XVERP to enable VERP
style delivery. The syntax allows two forms:

    MAIL FROM:<sender@domain> XVERP

    MAIL FROM:<sender@domain> XVERP=+=

The first form uses the default main.cf VERP delimiters, the second
form overrides them explicitly. The values shown are the recommended
ones.

VERP support in the Postfix sendmail command
============================================

The Postfix sendmail command has a -V flag to request VERP style
delivery.  Specify one of the following two forms:

    sendmail -V -f owner-listname ....

    sendmail -V+= -f owner-listname ....

The first form uses the default main.cf VERP delimiters, the second
form overrides them explicitly. The values shown are the recommended
ones.

VERP support in the Postfix QMQP server 
=======================================

When the Postfix QMQP server receives mail with an envelope sender
address of the form:

    listname-@your.domain-@[]

Postfix generates sender addresses listname-user=domain@your.domain,
using "-=" as the VERP delimiters because qmail/ezmlm expect this.