diff --git a/app/models/domain.rb b/app/models/domain.rb
index 498d579..fffaeba 100644
--- a/app/models/domain.rb
+++ b/app/models/domain.rb
@@ -34,6 +34,8 @@
 #  index_domains_on_uuid       (uuid)
 #
 
+require 'resolv'
+
 class Domain < ApplicationRecord
 
   include HasUUID
diff --git a/app/models/domain/dns_checks.rb b/app/models/domain/dns_checks.rb
index 1d5c0db..3d5c805 100644
--- a/app/models/domain/dns_checks.rb
+++ b/app/models/domain/dns_checks.rb
@@ -1,3 +1,5 @@
+require 'resolv'
+
 class Domain
 
   def dns_ok?
diff --git a/app/models/domain/dns_verification.rb b/app/models/domain/dns_verification.rb
index 8326e2e..b9831cb 100644
--- a/app/models/domain/dns_verification.rb
+++ b/app/models/domain/dns_verification.rb
@@ -1,3 +1,5 @@
+require 'resolv'
+
 class Domain
 
   def dns_verification_string
diff --git a/app/models/outgoing_message_prototype.rb b/app/models/outgoing_message_prototype.rb
index 2f98376..231bbf3 100644
--- a/app/models/outgoing_message_prototype.rb
+++ b/app/models/outgoing_message_prototype.rb
@@ -1,3 +1,5 @@
+require 'resolv'
+
 class OutgoingMessagePrototype
 
   attr_accessor :from
diff --git a/app/models/track_domain.rb b/app/models/track_domain.rb
index 26b87ef..88562bc 100644
--- a/app/models/track_domain.rb
+++ b/app/models/track_domain.rb
@@ -18,6 +18,8 @@
 #  excluded_click_domains :text(65535)
 #
 
+require 'resolv'
+
 class TrackDomain < ApplicationRecord
 
   include HasUUID
diff --git a/lib/postal/smtp_server/client.rb b/lib/postal/smtp_server/client.rb
index 947797b..d6e3aa5 100644
--- a/lib/postal/smtp_server/client.rb
+++ b/lib/postal/smtp_server/client.rb
@@ -1,3 +1,4 @@
+require 'resolv'
 require 'nifty/utils/random_string'
 
 module Postal