Postfix Address Verification Howto

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WARNING

The sender/recipient address verification feature described in this document is
suitable only for low-traffic sites. It performs poorly under high load and may
cause your site to be blacklisted by some providers. See the "Limitations"
section below for details.

What Postfix address verification can do for you

Address verification is a feature that allows the Postfix SMTP server to block
a sender (MAIL FROM) or recipient (RCPT TO) address until the address has been
verified to be deliverable.

The technique has obvious uses to reject junk mail with an unreplyable sender
address.

The technique may also be useful to block mail for undeliverable recipients,
for example on a mail relay host that does not have a list of all the valid
recipient addresses. This prevents undeliverable junk mail from entering the
queue, so that Postfix doesn't have to waste resources trying to send MAILER-
DAEMON messages back.

This feature is available in Postfix version 2.1 and later.

Topics covered in this document:

  * How address verification works
  * Limitations of address verification
  * Recipient address verification
  * Sender address verification for mail from frequently forged domains
  * Sender address verification for all email
  * Address verification database
  * Managing the address verification database
  * Controlling the routing of address verification probes
  * Forced probe routing examples
  * Limitations of forced probe routing

How address verification works

A sender or recipient address is verified by probing the nearest MTA for that
address, without actually delivering mail. The nearest MTA could be Postfix
itself, or it could be a remote MTA (SMTP interruptus). Probe messages are like
normal mail, except that they are never delivered, deferred or bounced; probe
messages are always discarded.

                Postfix         Postfix             Address
    Internet ->  SMTP   <->     verify      <->   verification
                server          server              database

                                   |    ^
                               probe    delivery
                            messages    status
                                   v    |

                            Postfix     Postfix
                             queue   -> delivery
                                         agents

With Postfix address verification turned on, normal mail will suffer only a
short delay of up to 6 seconds while an address is being 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
postconf(5) for details.

Limitations of address verification

  * When verifying a remote address, Postfix probes the nearest MTA for that
    address, without actually delivering mail to it. If the nearest MTA accepts
    the address, then Postfix assumes that the address is deliverable. In
    reality, mail for a remote address can bounce AFTER the nearest MTA accepts
    the recipient address.

  * Some sites may blacklist you when you are probing them too often (a probe
    is an SMTP session that does not deliver mail), or when you are probing
    them too often for a non-existent address. This is one reason why you
    should use sender address verification sparingly, if at all, when your site
    receives lots of email.

  * 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. This is SAFE because the Postfix SMTP server does not
    reject mail for this address.

    You can change this into the null address ("address_verify_sender ="). This
    is UNSAFE because address probes will fail with mis-configured sites that
    reject MAIL FROM: <>, while probes from "postmaster@$myorigin" would
    succeed.

Recipient address verification

As mentioned earlier, recipient address verification may be useful to block
mail for undeliverable recipients on a mail relay host that does not have a
list of all valid recipient addresses. This can help to prevent the mail queue
from filling up with MAILER-DAEMON messages.

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
        # Note 1: Be sure to read the "Caching" section below!
        # Note 2: Avoid hash files here. Use btree instead.
        address_verify_map = btree:/var/mta/verify

    /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.

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:

    /etc/postfix/main.cf:
        smtpd_sender_restrictions =
            permit_mynetworks
            ...
            check_sender_access hash:/etc/postfix/sender_access
            reject_unknown_sender_domain
            warn_if_reject reject_unverified_sender
            ...
        # Note 1: Be sure to read the "Caching" section below!
        # Note 2: Avoid hash files here. Use btree instead.
        address_verify_map = btree:/var/mta/verify

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 OK
        ...

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.

Address verification database

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 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 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 /var file system has sufficient space, try:

    /etc/postfix/main.cf:
        # Note: avoid hash files here. Use btree instead.
        address_verify_map = btree:/var/mta/verify

NOTE: Do not put this file in a file system that may run out of space. 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(8) 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 rename or
delete the file and run "postfix reload". The new verify daemon process will
then create a new 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 addresses
can be verified only when Postfix can access remote 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, as shown in the table below. Address classes are
defined in the ADDRESS_CLASS_README file.

                                                                               
    |Domain list            |Regular transport|Verify transport                |
    |                       |                 |                                |
    |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  |
    |                       |                 |                                |
    |(not applicable)       |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.

Forced probe routing 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 a network address translation box 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 of forced probe routing

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.

