Spam Filtering for Mail Exchangers:
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# A.3. Options and Settings

The main section of the Exim configuration file (before the first `begin` keyword) contains various macros, policy controls, and other general settings. Let us start by defining a couple of macros we will use later:

    # Define the message size limit; we will use this in the DATA ACL.
    MESSAGE_SIZE_LIMIT = 10M

    # Maximum message size for which we will run Spam or Virus scanning.
    # This is to reduce the load imposed on the server by very large messages.
    MESSAGE_SIZE_SPAM_MAX = 1M

    # Macro defining a secret that we will use to generate various hashes.
    # PLEASE CHANGE THIS!.
    SECRET = some-secret

Let us tweak some general Exim settings:

    # Treat DNS failures (SERVFAIL) as lookup failures.
    # This is so that we can later reject sender addresses
    # within non-existing domains, or domains for which no
    # nameserver exists.
    dns_again_means_nonexist = !+local_domains : !+relay_to_domains

    # Enable HELO verification in ACLs for all hosts
    helo_try_verify_hosts = *

    # Remove any limitation on the maximum number of incoming
    # connections we can serve at one time.  This is so that while
    # we later impose SMTP transaction delays for spammers, we
    # will not refuse to serve new connections.
    smtp_accept_max = 0

    # ..unless the system load is above 10
    smtp_load_reserve = 10

    # Do not advertise ESMTP "PIPELINING" to any hosts.
    # This is to trip up ratware, which often tries to pipeline
    # commands anyway.
    pipelining_advertise_hosts = :

Finally, we will point some Exim policy controls to five ACLs that we will create to evaluate the various stages of an incoming SMTP transaction:

    acl_smtp_connect = acl_connect
    acl_smtp_helo    = acl_helo
    acl_smtp_mail    = acl_mail_from
    acl_smtp_rcpt    = acl_rcpt_to
    acl_smtp_data    = acl_data

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