Postfix Address Verification

# Postfix Address Verification Howto

## WARNING

Recipient address verification may cause an increased load on down-stream servers in the case of a dictionary attack or a flood of backscatter bounces. Sender address verification may cause your site to be blacklisted by some providers. See also the "Limitations" section below for more.

## 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 is also 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 Postfix MTA verifies a sender or recipient address by probing the nearest MTA for that address, without actually delivering mail. The nearest MTA could be the Postfix MTA 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.

> | `->` | probe message | `->` | Postfix mail queue
> Internet | `->` | Postfix SMTP server | `<->` | Postfix verify server
> | `|` `v`
> `<-` | probe status | `<-` | Postfix delivery agents | `->` Local `->` Remote
> | `^` `|` `v`
> | Address verification database

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, or AFTER the nearest MTA accepts the message content.

- 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 an address when the nearest MTA for that address rejects mail from your machine for any reason. This is not a limitation, but it is mentioned here just in case people believe that it is a limitation.

- Unfortunately, some sites 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 a sender address "double-bounce@$myorigin" (with Postfix versions before 2.5, the default is "postmaster@$myorigin"). This is SAFE because the Postfix SMTP server does not reject mail for this address.

  You can change the probe sender address 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 "double-bounce@$myorigin" would succeed.

## Recipient address verification

As mentioned earlier, recipient address verification is 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. However, recipient address verification probes can increase the load on down-stream MTAs when you're being flooded by backscatter bounces, or when some spammer is mounting a dictionary attack.

By default, address verification results are saved in a persistent database (Postfix version 2.7 and later; with earlier versions, specify the database in main.cf as described later). The persistent database helps to avoid probing the same address repeatedly.

>     /etc/postfix/main.cf:
>         smtpd_recipient_restrictions =
>             permit_mynetworks
>             reject_unauth_destination
>             ...
>             reject_unknown_recipient_domain
>             reject_unverified_recipient
>             ...
>         # Postfix 2.6 and later privacy feature.
>         # unverified_recipient_reject_reason = Address lookup failed

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 the numerical Postfix SMTP server reply code when a recipient address is known to bounce. Change this setting into 550 when you trust Postfix's judgments.

The following features are available in Postfix 2.6 and later.

The unverified_recipient_defer_code parameter (default 450) specifies the numerical Postfix SMTP server reply code when a recipient address probe fails with some temporary error. Some sites insist on changing this into 250. NOTE: This change turns MX servers into backscatter sources when the load is high.

The unverified_recipient_reject_reason parameter (default: empty) specifies fixed text that Postfix will send to remote SMTP clients, instead of sending actual address verification details. Do not specify the SMTP status code or enhanced status code.

The unverified_recipient_tempfail_action parameter (default: defer_if_permit) specifies the Postfix SMTP server action when a recipient address verification probe fails with some temporary error.

## Sender address verification for mail from frequently forged domains

Only for very small sites, 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
>         # Postfix 2.6 and later.
>         # unverified_sender_defer_code = 250

>         # Default setting for Postfix 2.7 and later.
>         # Note 1: Be sure to read the "Caching" section below!
>         # Note 2: Avoid hash files here. Use btree instead.
>         address_verify_map = btree:/var/lib/postfix/verify

>     /etc/postfix/sender_access:
>         # Don't do this when you handle lots of email.
>         aol.com     reject_unverified_sender
>         hotmail.com reject_unverified_sender
>         bigfoot.com reject_unverified_sender
>         ... etcetera ...

At some point in cyberspace/time, a list of frequently forged MAIL FROM domains could 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
>             ...
>         # Postfix 2.6 and later.
>         # unverified_sender_reject_reason = Address verification failed

>         # Default setting for Postfix 2.7 and later.
>         # Note 1: Be sure to read the "Caching" section below!
>         # Note 2: Avoid hash files here. Use btree instead.
>         address_verify_map = btree:/var/lib/postfix/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 the numerical Postfix server reply code when a sender address is known to bounce. Change this setting into 550 when you trust Postfix's judgments.

The following features are available in Postfix 2.6 and later.

The unverified_sender_defer_code parameter (default 450) specifies the numerical Postfix SMTP server reply code when a sender address verification probe fails with some temporary error. Specify a valid 2xx or 4xx code.

The unverified_sender_reject_reason parameter (default: empty) specifies fixed text that Postfix will send to remote SMTP clients, instead of sending actual addres verification details. Do not specify the SMTP status code or enhanced status code.

The unverified_sender_tempfail_action parameter (default: defer_if_permit) specifies the Postfix SMTP server action when a sender address verification probe fails with some temporary error.

## Address verification database

To improve performance, the Postfix verify(8) daemon can save address verification results to a persistent database. This is enabled by default with Postfix 2.7 and later. The address_verify_map (NOTE: singular) configuration parameter specifies persistent storage for sender or recipient address verification results. If you specify an empty value, all address verification results are lost after "postfix reload" or "postfix stop".

>     /etc/postfix/main.cf:
>         # Default setting for Postfix 2.7 and later.
>         # Note: avoid hash files here. Use btree instead.
>         address_verify_map = btree:$data_directory/verify_cache

>         # Default setting for Postfix 2.6 and earlier.
>         # This uses non-persistent storage only.
>         address_verify_map =

NOTE 1: The database file should be stored under a Postfix-owned directory, such as $data_directory.

> As of version 2.5, Postfix no longer uses root privileges when opening this file. To maintain backwards compatibility, an attempt to open the file under a non-Postfix directory is redirected to the Postfix-owned data_directory, and a warning is logged. If you wish to continue using a pre-existing database file, change its file ownership to the account specified with the mail_owner parameter, and either move the file to the data_directory, or move it to some other Postfix-owned directory.

NOTE 2: 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.

NOTE 3: The verify(8) daemon will create a new database when none exists. It will open or create the file before entering the chroot jail.

## Managing the address verification database

The verify(8) manual page describes parameters that control how long address verification results are cached before they need to be refreshed, and how long results can remain "unrefreshed" before they expire. Postfix uses different controls for positive results (address was accepted) and for negative results (address was rejected, or address verification failed for some other reason).

The verify(8) daemon will periodically remove expired entries from the address verification database, and log the number of entries retained and dropped (Postfix versions 2.7 and later). A cleanup run is logged as "partial" when the daemon terminates early because of "postfix reload, "postfix stop", or because the daemon received no requests for $max_idle seconds. Postfix versions 2.6 and earlier do not implement automatic address verification database cleanup. There, the database is managed manually as described next.

When the address verification database file becomes too big, or when it becomes corrupted, the solution is to manually rename or delete (NOT: truncate) the file and run "postfix reload". The verify(8) daemon will then create a new database file.

## 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. The address_verify_sender_dependent_relayhost_maps parameter does the same for sender-dependent relayhost selection.

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.