Address verification
====================

Sender address verification blocks mail from an unknown sender
address until after the address is verified.  An address is verified
by probing the nearest MTA for that address, without actually
delivering mail to it (SMTP interruptus). Probe messages are like
normal mail, but are discarded instead of being deferred or bounced.

The same technique may be useful to block mail for undeliverable
recipients, for example on mail relay hosts that do not have a copy
of all the relayed recipient addresses.

With address verification turned on, normal mail will suffer only
a short delay of up to 6 seconds while an address is verified for
the first time.  Once an address status is known, the status is
cached and Postfix replies immediately.  When verification takes
too long the Postfix SMTP server defers the sender or recipient
address with a 450 reply. Normal mail clients will connect again
after some delay.

The address verification delay is configurable with the main.cf
address_verify_poll_count and address_verify_poll_delay parameters.
See the sample-verify.cf file for details.

Limitations
===========

Postfix probes the nearest MTA for the address that is being
verified, without actually sending mail to that address. If the
nearest MTA accepts the recipient, then Postfix assumes that the
address is deliverable, even when the address will bounce AFTER
that MTA accepts it.

Normally, address verification probe messages follow the same path
as regular mail.  However, some sites send mail to the Internet
via an intermediate relayhost; this breaks address verification.
See below, section "Controlling the routing of address verification
probes", for how to override mail routing and for possible limitations
when you have to do this.

Postfix assumes that an address is undeliverable when the nearest
MTA for the address rejects the probe, regardless of the reason
for rejection (client rejected, HELO rejected, MAIL FROM rejected,
etc.).  Thus, Postfix rejects mail when the sender's MTA rejects
mail from your machine.  This is a good thing.

Unfortunately, some major sites such as YAHOO do not reject unknown
addresses in reply to the RCPT TO command, but report a delivery
failure in response to "end of data" after a message is transferred.
Postfix address verification does not work with such sites.

By default, Postfix probe messages have "postmaster@$myorigin" as
the sender address. You can change this into the null address
(address_verify_sender =) but that causes address probes to fail
with mis-configured sites that reject MAIL FROM: <>.

Turning on recipient address verification
=========================================

Recipient address verification may be useful to block mail for
undeliverable recipients on mail relay hosts that do not have a
copy of all the relayed recipient addresses. This prevents the mail
queue from filling up with undeliverable and bounced SPAM.

Recipient address verification is relatively straightforward and
there are no surprises. If a recipient probe fails, then Postfix
rejects mail for the recipient address.  If a recipient probe
succeeds, then Postfix accepts mail for the recipient address.

/etc/postfix/main.cf:
    smtpd_recipient_restrictions = 
	permit_mynetworks
	reject_unauth_destination
	...
	reject_unknown_recipient_domain
	reject_unverified_recipient
	...

The "reject_unknown_recipient_domain" restriction blocks mail for
non-existent domains. Putting this before "reject_unverified_recipient"
avoids the overhead of generating unnecessary probe messages.

The unverified_recipient_reject_code parameter (default 450)
specifies how Postfix replies when a recipient address is known to
bounce.  Change this setting into 550 when you trust Postfix's
judgments.

Sender address verification for mail from frequently forged domains
===================================================================

It is relatively safe to turn on sender address verification for
specific domains that often appear in forged email.

/etc/postfix/main.cf:
    smtpd_sender_restrictions = hash:/etc/postfix/sender_access
    unverified_sender_reject_code = 550
 
/etc/postfix/sender_access:
    aol.com     reject_unverified_sender
    hotmail.com reject_unverified_sender
    bigfoot.com reject_unverified_sender
    ... etcetera ...

A list of frequently forged MAIL FROM domains can be found at
http://www.monkeys.com/anti-spam/filtering/sender-domain-validate.in

NOTE: One of the first things you might want to do is to turn on
sender address verification for all your own domains.

Turning on sender address verification for all email
====================================================

Unfortunately, sender address verification cannot simply be turned
on for all email - you are likely to lose legitimate mail from
mis-configured systems. You almost certainly will have to set up
white lists for specific addresses, or even for entire domains.

To find out how sender address verification would affect your mail,
specify "warn_if_reject reject_unverified_sender" so that you can
see what mail would be blocked:

    smtpd_sender_restrictions = 
	permit_mynetworks
	... 
	check_sender_access hash:/etc/postfix/sender_access
	reject_unknown_sender_domain
	warn_if_reject reject_unverified_sender 
	...

This is also a good way to populate your cache with address
verification results before you start to actually reject mail.

The sender_access restriction is needed to whitelist domains or
addresses that are known to be OK.  Although Postfix will not mark
a known-to-be-good address as bad after a probe fails, it is better
to be safe than sorry.

NOTE: You will have to whitelist sites such as securityfocus.com
and other sites that operate mailing lists that use a different
sender address for each posting (VERP).  Such addresses pollute
the address verification cache quickly, and generate unnecessary
sender verification probes.

/etc/postfix/sender_access
    securityfocus.com
    ...

The "reject_unknown_sender_domain" restriction blocks mail from
non-existent domains. Putting this before "reject_unverified_sender"
avoids the overhead of generating unnecessary probe messages.

The unverified_sender_reject_code parameter (default 450) specifies
how Postfix replies when a sender address is known to bounce.
Change this setting into 550 when you trust Postfix's judgments.

Caching
=======

NOTE: By default, address verification information is not stored
in a persistent file. You have to specify one in main.cf (see
below). Persistent storage is off by default because it may need
more disk space than is available in your root file system.

Address verification information is cached by the Postfix verify
daemon.  Postfix has a bunch of parameters that control the caching
of positive and negative results. Refer to the verify(8) manual
page or the sample-verify.cf file for details.

The address_verify_map (NOTE: singular) configuration parameter
specifies an optional persistent database for sender address
verification results.  If you don't specify a file, all address
verification information is lost after "postfix reload" or "postfix
stop".

If your root file system has sufficient space, try:

    address_verify_map = btree:/etc/postfix/verify

NOTE: Do not put this file in a file system that fills up. When
the address verification table gets corrupted the world comes to
an end and YOU will have to MANUALLY fix things as described in
the next section. Meanwhile you will not receive mail via SMTP.

The verify daemon process will create a new database when none
exists, and will open/create the file before it enters the chroot
jail and before it drops root privileges.

Managing the address verification database
==========================================

The verify(8) manual page describes parameters that control how
long information remains cached before it needs to be refreshed,
and how long information can remain "unrefreshed" before it expires.
Postfix uses different controls for positive results (address was
accepted) and for negative results (address was rejected).

Right now, no tools are provided to manage the address verification
database. If the file gets too big, or if it gets corrupted, you
can manually delete the file and run "postfix reload".  The new
verify daemon process will then create a new, empty, database.

Controlling the routing of address verification probes
======================================================

By default, Postfix sends address verification probe messages via
the same route as regular mail, because that normally produces the
most accurate result. It's no good to verify a local address by
connecting to your own SMTP port; that just triggers all kinds of
mailer loop alarms. The same is true for any destination that your
machine is best MX host for:  hidden domains, virtual domains, etc.

However, some sites have a complex infrastructure where mail is
not sent directly to the Internet, but is instead given to an
intermediate relayhost. This is a problem for address verification,
because remote Internet destinations can be verified only when
Postfix can access those destinations directly.

For this reason, Postfix allows you to override the routing parameters
when it delivers an address verification probe message.

First, the address_verify_relayhost parameter allows you to override
the relayhost setting, and the address_verify_transport_maps
parameter allows you to override the transport_maps setting.

Second, each address class is given its own address verification
version of the message delivery transport (address classes are
defined in the ADDRESS_CLASS_README file) as shown in the table:

Destination type        Regular transport    Verify transport
                        parameter name       parameter name
=====================================================================
mydestination           local_transport      address_verify_local_transport
virtual_alias_domains   (not applicable)     (not applicable)
virtual_mailbox_domains virtual_transport    address_verify_virtual_transport
relay_domains           relay_transport      address_verify_relay_transport
other                   default_transport    address_verify_default_transport

By default, the parameters that control delivery of address probes
have the same value as the parameters that control normal mail
delivery.

Examples
--------

In a typical scenario one would override the relayhost setting 
for address verification probes and leave everything else alone:

/etc/postfix/main.cf:
    relayhost = $mydomain
    address_verify_relayhost =

Sites behind an address translation relay might have to use a
different SMTP client that sends the correct hostname information:

/etc/postfix/main.cf:
    relayhost = $mydomain
    address_verify_relayhost =
    address_verify_default_transport = direct_smtp

/etc/postfix/master.cf:
    direct_smtp .. .. .. ..  .. .. .. .. .. smtp -o smtp_helo_name=nat.box.tld

Limitations
-----------

Inconsistencies can happen when probe messages don't follow the
same path as regular mail.  For example, a message can be accepted
when it follows the regular route while an otherwise identical
probe message is rejected when it follows the forced route. The
opposite can happen, too, but is less likely.
