create_resources

create_resources("mail::domain", $domains)

data.json

{
  "relay_domains": {
    "x.net": {
      "nexthop": "70.x.x.x",
      "spamdestination": "rip@devco.net",
      "spamthreshold": 1500,
      "enable_antispam": 1
    },
    "x.co.uk": {
      "nexthop": "70.x.x.x",
      "spamdestination": "rip@devco.net",
      "spamthreshold": 1500,
      "enable_antispam": 1
    },
}

puppet3_with_helper

$names = keys($domains)

mail::domains($names: domains => $domains}

define mail::domains($domains) {
   $domain = $domains[$name]
  
   mail::domain{$name:
     nexthop => $domain["nexthop"]
     .
     .
   }
}

puppet4_explicit

$domains.each |$name, $domain| { 
  mail::domain{$name:
    nexthop => $domain["nexthop"]
    .
    .
  }
}

puppet4_wildcard

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

puppet4_wildcard_with_defaults

$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 
  }
}

puppet4_wildcard_with_resource_defaults

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

$domains.each |$name, $domain| { 
  mail::domain{
    default:
      * => $defaults;
    
    $name:
      * => $domain
  }
}