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    Used to parse email addresses from header fields like To/From/cc, per
    RFC 5322.  If installed, it may additionally be used by internal parser
    to process complex lists.

  - Mail::DKIM (from CPAN)

    If this module is installed, and you enable the DKIM plugin,
    SpamAssassin will perform DKIM lookups when a DKIM-Signature header is
    present in the message headers.  Current versions of Mail::DKIM (0.20 or
    later) also perform Domain Key lookups on DomainKey-Signature headers,
    without requiring the Mail::DomainKeys module, which is now obsolete. 
    Version 0.37 or later is preferred, the absolute minimal version is
    0.31.
    
  - Mail::SPF (from CPAN)

    Used to check DNS Sender Policy Framework (SPF) records to fight email
    address forgery and make it easier to identify spams.

  - MaxMind::DB::Reader::XS (GeoIP2) (from CPAN)
  - MaxMind::DB::Reader (GeoIP2) (from CPAN)
  - IP::Geolocation::MMDB (GeoIP2) (from CPAN)
  - IP::Country::DB_File (from CPAN)
  - Geo::IP (old deprecated GeoIP) (from CPAN)
  - IP::Country::Fast (deprecated) (from CPAN)

    Geolocation modules, choose one from the list (in recommended order).

    Used by the RelayCountry plugin (not enabled by default) to determine
    the domain country codes of each relay in the path of an email.  Also
    used by the URILocalBL plugin (not enabled by default) to provide ISP
    and Country code based filtering.

    See: https://wiki.apache.org/spamassassin/RelayCountryPlugin

  - Mail::DMARC

    Used by the optional DMARC check plugin, which itself requires DKIM and
    SPF features working.

  - DB_File (from CPAN)

    Used to store data on-disk, for the Bayes-style logic, TxRep, and
    auto-welcomelist.  *Much* more efficient than the other standard Perl
    database packages.  Strongly recommended.

    There seems to be a bug in libdb 4.1.25, which is
    distributed by default on some versions of Linux.  See
    https://wiki.apache.org/spamassassin/DbFileSleepBug for details.

  - IO::Socket::IP (from CPAN)
  - IO::Socket::INET6 (from CPAN)

    Installing IO::Socket::IP is recommended if spamd is to listen on IPv6
    sockets or if DNS queries should go to an IPv6 name server.  If
    IO::Socket::IP is not available, using a deprecated module
    IO::Socket::INET6 will be attempted, and in its absence the support for
    IPv6 will not be available.  Some plugins and underlying modules may
    also prefer IO::Socket::IP over IO::Socket::INET6.

  - IO::Socket::SSL (from CPAN)

    If you wish to use SSL encryption to communicate between spamc and spamd
    (the --ssl option to spamd), you need to install this module.  (You will
    need the OpenSSL libraries and use the ENABLE_SSL=yes argument to
    Makefile.PL to build and run an SSL compatible spamc.)

  - Net::Patricia

    If this module is available, it will be used for IP address lookups in
    tables internal_networks, trusted_networks, and msa_networks. 
    Recommended when a number of entries in these tables is hundred or more. 
    However, in case of overlapping (or conflicting) networks in these
    tables, lookup results may differ as Net::Patricia finds a
    tightest-matching entry, while a sequential NetAddr::IP search finds a
    first-matching entry.  So when overlapping network ranges are given,
    specifying more specific subnets (longest netmask) first, followed by
    wider subnets ensures predictable results.

  - DBI *and* DBD driver/modules for your database (from CPAN)

    If you intend to use SpamAssassin with an SQL database backend for user
    configuration data, Bayes storage, or other storage, you will need to
    have these installed; both the basic DBI module and the driver for your
    database (for example DBD::MariaDB, DBD::mysql or DBD::Pg).

  - Archive::Zip
  - IO::String

    Required by the optional OLEVBMacro plugin.

  - Razor2

    If you plan to use Vipul's Razor, note that versions up to and including
    version 2.82 include a bug that will slow down the entire perl
    interpreter.  Version 2.83 or later fixes this.

    If you do not plan to use this plugin, be sure to comment out its
    loadplugin line in "/etc/mail/spamassassin/v310.pre".

  - Digest::SHA1 (from CPAN)

    An external perl module razor-agents-2.84 as used by a Razor2 plugin
    seems to be the only remaining component depending on Digest::SHA1 (note
    that a packager may ship a patched version of razor-agents which can use
    Digest::SHA instead)

  - LWP::UserAgent (aka libwww-perl) (from CPAN)

    Can be used by sa-update to retrieve update archives, as alternative to
    curl/wget/fetch.

  - Net::SMTP (from CPAN)

    Used when manually reporting spam to SpamCop.

Examples of installing most recommended modules on popular distributions:

 Debian/Ubuntu:
  apt-get install libencode-detect-perl libnet-libidn-perl \
    libemail-address-xs-perl libmail-dkim-perl libmail-spf-perl \