Spam Filtering for Mail Exchangers:
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# A.4. Building the ACLs - First Pass

In the acl section (following `begin acl`), we need to define these ACLs. In doing so, we will incorporate some of the basic Techniques described earlier in this document, namely DNS checks and SMTP checks.

In this pass, we will do most of the checks in acl_rcpt_to, and leave the other ACLs largely empty. That is because most of the commonly used ratware does not understand rejections early in the SMTP transaction - it keeps trying. On the other hand, most ratware clients give up if the RCPT TO: fails.

We create all these ACLs, however, because we will use them later.

## A.4.1. acl_connect

    # This access control list is used at the start of an incoming
    # connection.  The tests are run in order until the connection
    # is either accepted or denied.

    acl_connect:

      # In this pass, we do not perform any checks here.
      accept

## A.4.2. acl_helo

    # This access control list is used for the HELO or EHLO command in
    # an incoming SMTP transaction.  The tests are run in order until the
    # greeting is either accepted or denied.

    acl_helo:

      # In this pass, we do not perform any checks here.
      accept

## A.4.3. acl_mail_from

    # This access control list is used for the MAIL FROM: command in an
    # incoming SMTP transaction.  The tests are run in order until the
    # sender address is either accepted or denied.
    #

    acl_mail_from:

      # Accept the command.
      accept

## A.4.4. acl_rcpt_to

    # This access control list is used for every RCPT command in an
    # incoming SMTP message.  The tests are run in order until the
    # recipient address is either accepted or denied.

    acl_rcpt_to:

      # Accept mail received over local SMTP (i.e. not over TCP/IP).
      # We do this by testing for an empty sending host field.
      # Also accept mails received from hosts for which we relay mail.
      #
      # Recipient verification is omitted here, because in many
      # cases the clients are dumb MUAs that don't cope well with
      # SMTP error responses.
      #
      accept
        hosts       = : +relay_from_hosts

      # Accept if the message arrived over an authenticated connection,
      # from any host. Again, these messages are usually from MUAs, so
      # recipient verification is omitted.
      #
      accept
        authenticated = *

      ######################################################################
      # DNS checks
      ######################################################################
      #
      # The results of these checks are cached, so multiple recipients
      # does not translate into multiple DNS lookups.
      #

      # If the connecting host is in one of a select few DNSbls, then
      # reject the message.  Be careful when selecting these lists; many
      # would cause a large number of false postives, and/or have no
      # clear removal policy.
      #
      deny
        dnslists    = dnsbl.sorbs.net : \
                      dnsbl.njabl.org : \
                      cbl.abuseat.org : \
                      bl.spamcop.net
        message     = $sender_host_address is listed in $dnslist_domain\
                      ${if def:dnslist_text { ($dnslist_text)}}

      # If reverse DNS lookup of the sender's host fails (i.e. there is
      # no rDNS entry, or a forward lookup of the resulting name does not
      # match the original IP address), then reject the message.
      #
      deny
        message     = Reverse DNS lookup failed for host $sender_host_address.
        !verify     = reverse_host_lookup

      ######################################################################
      # Hello checks
      ######################################################################

      # If the remote host greets with an IP address, then reject the mail.
      #
      deny
        message     = Message was delivered by ratware
        log_message = remote host used IP address in HELO/EHLO greeting
        condition   = ${if isip {$sender_helo_name}{true}{false}}

      # Likewise if the peer greets with one of our own names
      #
      deny
        message     = Message was delivered by ratware
        log_message = remote host used our name in HELO/EHLO greeting.
        condition   = ${if match_domain{$sender_helo_name}\
                           {$primary_hostname:+local_domains:+relay_to_domains}\
                           {true}{false}}

      deny
        message     = Message was delivered by ratware
        log_message = remote host did not present HELO/EHLO greeting.
        condition   = ${if def:sender_helo_name {false}{true}}

      # If HELO verification fails, we add a X-HELO-Warning: header in
      # the message.
      #
      warn
        message     = X-HELO-Warning: Remote host $sender_host_address \
                      ${if def:sender_host_name {($sender_host_name) }}\
                      incorrectly presented itself as $sender_helo_name
        log_message = remote host presented unverifiable HELO/EHLO greeting.
        !verify     = helo

      ######################################################################
      # Sender Address Checks
      ######################################################################

      # If we cannot verify the sender address, deny the message.
      #
      # You may choose to remove the "callout" option.  In particular,
      # if you are sending outgoing mail through a smarthost, it will not
      # give any useful information.
      #
      # Details regarding the failed callout verification attempt are
      # included in the 550 response; to omit these, change
      # "sender/callout" to "sender/callout,no_details".
      #
      deny
        message     = <$sender_address> does not appear to be a \
                      valid sender address.
        !verify     = sender/callout

      ######################################################################
      # Recipent Address Checks
      ######################################################################

      # Deny if the local part contains @ or % or / or | or !. These are
      # rarely found in genuine local parts, but are often tried by people
      # looking to circumvent relaying restrictions.
      #
      # Also deny if the local part starts with a dot. Empty components
      # aren't strictly legal in RFC 2822, but Exim allows them because
      # this is common.  However, actually starting with a dot may cause
      # trouble if the local part is used as a file name (e.g. for a
      # mailing list).
      #
      deny
        local_parts = ^.*[@%!/|] : ^\\.

      # Drop the connection if the envelope sender is empty, but there is
      # more than one recipient address.  Legitimate DSNs are never sent
      # to more than one address.
      #
      drop
        message      = Legitimate bounces are never sent to more than one \
                       recipient.
        senders      = : postmaster@*
        condition    = $recipients_count

      # Reject the recipient address if it is not in a domain for
      # which we are handling mail.
      #
      deny
        message     = relay not permitted
        !domains    = +local_domains : +relay_to_domains

      # Reject the recipient if it is not a valid mailbox.
      # If the mailbox is not on our system (e.g. if we are a
      # backup MX for the recipient domain), then perform a
      # callout verification; but if the destination server is
      # not responding, accept the recipient anyway.
      #
      deny
        message     = unknown user
        !verify     = recipient/callout=20s,defer_ok

      # Otherwise, the recipient address is OK.
      #
      accept

## A.4.5. acl_data

    # This access control list is used for message data received via
    # SMTP.  The tests are run in order until the recipient address
    # is either accepted or denied.

    acl_data:

      # Add Message-ID if missing in messages received from our own hosts.
      warn
        condition   = ${if !def:h_Message-ID: {1}}
        hosts       = : +relay_from_hosts
        message     = Message-ID: <E$message_id@$primary_hostname>

      # Accept mail received over local SMTP (i.e. not over TCP/IP).
      # We do this by testing for an empty sending host field.
      # Also accept mails received from hosts for which we relay mail.
      #
      accept
        hosts       = : +relay_from_hosts

      # Accept if the message arrived over an authenticated connection, from
      # any host.
      #
      accept
        authenticated = *

      # Enforce a message-size limit
      #
      deny
        message     = Message size $message_size is larger than limit of \
                      MESSAGE_SIZE_LIMIT
        condition   = ${if >{$message_size}{MESSAGE_SIZE_LIMIT}{true}{false}}

      # Deny unless the address list header is syntactically correct.
      #
      deny
        message     = Your message does not conform to RFC2822 standard
        log_message = message header fail syntax check
        !verify     = header_syntax

      # Deny non-local messages with no Message-ID, or no Date
      #
      # Note that some specialized MTAs, such as certain mailing list
      # servers, do not automatically generate a Message-ID for bounces.
      # Thus, we add the check for a non-empty sender.
      #
      deny
        message     = Your message does not conform to RFC2822 standard
        log_message = missing header lines
        !hosts      = +relay_from_hosts
        !senders    = : postmaster@*
        condition   = ${if or {{!def:h_Message-ID:}\
                               {!def:h_Date:}\
                               {!def:h_Subject:}} {true}{false}}

      # Warn unless there is a verifiable sender address in at least
      # one of the "Sender:", "Reply-To:", or "From:" header lines.
      #
      warn
        message     = X-Sender-Verify-Failed: No valid sender in message header
        log_message = No valid sender in message header
        !verify     = header_sender

      # Accept the message.
      #
      accept

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