$defaults = {
  "spamthreshold" => 1500,
  "enable_antispam" => 1
}

$domains.each |$name, $domain| { 
  # see https://docs.puppetlabs.com/puppet/4.2/reference/lang_resources_advanced.html#setting-attributes-from-a-hash
  mail::domain{$name:
    * => $defaults + $domain  # + now merge hashes 
  }
}