diff --git a/lib/diaspora_federation/entities/reshare.rb b/lib/diaspora_federation/entities/reshare.rb
index 96ae593..ff235fa 100644
--- a/lib/diaspora_federation/entities/reshare.rb
+++ b/lib/diaspora_federation/entities/reshare.rb
@@ -10,7 +10,7 @@ module DiasporaFederation
       #   The diaspora* ID of the person who posted the original post
       #   @see Person#author
       #   @return [String] diaspora* ID
-      property :root_author, :string, xml_name: :root_diaspora_id
+      property :root_author, :string, optional: true, xml_name: :root_diaspora_id
 
       # @!attribute [r] root_guid
       #   Guid of the original post
diff --git a/lib/diaspora_federation/validators/account_deletion_validator.rb b/lib/diaspora_federation/validators/account_deletion_validator.rb
index 872472b..60a32cf 100644
--- a/lib/diaspora_federation/validators/account_deletion_validator.rb
+++ b/lib/diaspora_federation/validators/account_deletion_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class AccountDeletionValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
     end
   end
 end
diff --git a/lib/diaspora_federation/validators/account_migration_validator.rb b/lib/diaspora_federation/validators/account_migration_validator.rb
index f0dd336..1c11b25 100644
--- a/lib/diaspora_federation/validators/account_migration_validator.rb
+++ b/lib/diaspora_federation/validators/account_migration_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class AccountMigrationValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :profile, :not_nil
     end
diff --git a/lib/diaspora_federation/validators/contact_validator.rb b/lib/diaspora_federation/validators/contact_validator.rb
index c22ecc7..3c88805 100644
--- a/lib/diaspora_federation/validators/contact_validator.rb
+++ b/lib/diaspora_federation/validators/contact_validator.rb
@@ -4,8 +4,8 @@ module DiasporaFederation
     class ContactValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
-      rule :recipient, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
+      rule :recipient, :diaspora_id
       rule :following, :boolean
       rule :sharing, :boolean
     end
diff --git a/lib/diaspora_federation/validators/conversation_validator.rb b/lib/diaspora_federation/validators/conversation_validator.rb
index e8c84ee..43e68cb 100644
--- a/lib/diaspora_federation/validators/conversation_validator.rb
+++ b/lib/diaspora_federation/validators/conversation_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class ConversationValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
       rule :guid, :guid
 
       rule :subject, [:not_empty, length: {maximum: 255}]
diff --git a/lib/diaspora_federation/validators/event_validator.rb b/lib/diaspora_federation/validators/event_validator.rb
index ab926a4..a6c0aa6 100644
--- a/lib/diaspora_federation/validators/event_validator.rb
+++ b/lib/diaspora_federation/validators/event_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class EventValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :guid, :guid
 
diff --git a/lib/diaspora_federation/validators/message_validator.rb b/lib/diaspora_federation/validators/message_validator.rb
index 1e757f6..6ded966 100644
--- a/lib/diaspora_federation/validators/message_validator.rb
+++ b/lib/diaspora_federation/validators/message_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class MessageValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
       rule :guid, :guid
       rule :conversation_guid, :guid
 
diff --git a/lib/diaspora_federation/validators/participation_validator.rb b/lib/diaspora_federation/validators/participation_validator.rb
index 13b8c1e..c3c4917 100644
--- a/lib/diaspora_federation/validators/participation_validator.rb
+++ b/lib/diaspora_federation/validators/participation_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class ParticipationValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
       rule :guid, :guid
       rule :parent_guid, :guid
       rule :parent_type, [:not_empty, regular_expression: {regex: /\APost\z/}]
diff --git a/lib/diaspora_federation/validators/person_validator.rb b/lib/diaspora_federation/validators/person_validator.rb
index 62cac9d..ddfad99 100644
--- a/lib/diaspora_federation/validators/person_validator.rb
+++ b/lib/diaspora_federation/validators/person_validator.rb
@@ -6,7 +6,7 @@ module DiasporaFederation
 
       rule :guid, :guid
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :url, %i[not_nil URI]
 
diff --git a/lib/diaspora_federation/validators/photo_validator.rb b/lib/diaspora_federation/validators/photo_validator.rb
index 48e0d73..7c18107 100644
--- a/lib/diaspora_federation/validators/photo_validator.rb
+++ b/lib/diaspora_federation/validators/photo_validator.rb
@@ -6,7 +6,7 @@ module DiasporaFederation
 
       rule :guid, :guid
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :public, :boolean
 
diff --git a/lib/diaspora_federation/validators/related_entity_validator.rb b/lib/diaspora_federation/validators/related_entity_validator.rb
index 1f2ffe3..17c2726 100644
--- a/lib/diaspora_federation/validators/related_entity_validator.rb
+++ b/lib/diaspora_federation/validators/related_entity_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class RelatedEntityValidator < Validation::Validator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
       rule :local, :boolean
       rule :public, :boolean
     end
diff --git a/lib/diaspora_federation/validators/relayable_validator.rb b/lib/diaspora_federation/validators/relayable_validator.rb
index f32b384..618f3ce 100644
--- a/lib/diaspora_federation/validators/relayable_validator.rb
+++ b/lib/diaspora_federation/validators/relayable_validator.rb
@@ -6,7 +6,7 @@ module DiasporaFederation
       # @param [Validation::Validator] validator the validator in which it is included
       def self.included(validator)
         validator.class_eval do
-          rule :author, %i[not_empty diaspora_id]
+          rule :author, :diaspora_id
           rule :guid, :guid
           rule :parent_guid, :guid
           rule :parent, :not_nil
diff --git a/lib/diaspora_federation/validators/reshare_validator.rb b/lib/diaspora_federation/validators/reshare_validator.rb
index db8858d..b28621d 100644
--- a/lib/diaspora_federation/validators/reshare_validator.rb
+++ b/lib/diaspora_federation/validators/reshare_validator.rb
@@ -8,7 +8,7 @@ module DiasporaFederation
 
       rule :root_guid, :guid
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :guid, :guid
 
diff --git a/lib/diaspora_federation/validators/retraction_validator.rb b/lib/diaspora_federation/validators/retraction_validator.rb
index de70bc8..922b44f 100644
--- a/lib/diaspora_federation/validators/retraction_validator.rb
+++ b/lib/diaspora_federation/validators/retraction_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class RetractionValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :target_guid, :guid
       rule :target_type, :not_empty
diff --git a/lib/diaspora_federation/validators/status_message_validator.rb b/lib/diaspora_federation/validators/status_message_validator.rb
index 43a13be..62eee98 100644
--- a/lib/diaspora_federation/validators/status_message_validator.rb
+++ b/lib/diaspora_federation/validators/status_message_validator.rb
@@ -4,7 +4,7 @@ module DiasporaFederation
     class StatusMessageValidator < OptionalAwareValidator
       include Validation
 
-      rule :author, %i[not_empty diaspora_id]
+      rule :author, :diaspora_id
 
       rule :guid, :guid
 
diff --git a/spec/lib/diaspora_federation/validators/account_deletion_validator_spec.rb b/spec/lib/diaspora_federation/validators/account_deletion_validator_spec.rb
index 7d0b542..2c76af6 100644
--- a/spec/lib/diaspora_federation/validators/account_deletion_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/account_deletion_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
   end
 end
diff --git a/spec/lib/diaspora_federation/validators/account_migration_validator_spec.rb b/spec/lib/diaspora_federation/validators/account_migration_validator_spec.rb
index eca0e5d..7d95768 100644
--- a/spec/lib/diaspora_federation/validators/account_migration_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/account_migration_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     describe "#person" do
diff --git a/spec/lib/diaspora_federation/validators/contact_validator_spec.rb b/spec/lib/diaspora_federation/validators/contact_validator_spec.rb
index 476d859..27ab84a 100644
--- a/spec/lib/diaspora_federation/validators/contact_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/contact_validator_spec.rb
@@ -8,7 +8,6 @@ module DiasporaFederation
       describe "##{prop}" do
         it_behaves_like "a diaspora* ID validator" do
           let(:property) { prop }
-          let(:mandatory) { true }
         end
       end
     end
diff --git a/spec/lib/diaspora_federation/validators/conversation_validator_spec.rb b/spec/lib/diaspora_federation/validators/conversation_validator_spec.rb
index 7891159..49670ff 100644
--- a/spec/lib/diaspora_federation/validators/conversation_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/conversation_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     it_behaves_like "a guid validator" do
diff --git a/spec/lib/diaspora_federation/validators/event_validator_spec.rb b/spec/lib/diaspora_federation/validators/event_validator_spec.rb
index 1bd3864..c516a95 100644
--- a/spec/lib/diaspora_federation/validators/event_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/event_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     it_behaves_like "a guid validator" do
diff --git a/spec/lib/diaspora_federation/validators/message_validator_spec.rb b/spec/lib/diaspora_federation/validators/message_validator_spec.rb
index 7347b9b..0889cbb 100644
--- a/spec/lib/diaspora_federation/validators/message_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/message_validator_spec.rb
@@ -5,7 +5,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     describe "#guid" do
diff --git a/spec/lib/diaspora_federation/validators/participation_validator_spec.rb b/spec/lib/diaspora_federation/validators/participation_validator_spec.rb
index cd7cadd..7ffdcfc 100644
--- a/spec/lib/diaspora_federation/validators/participation_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/participation_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     describe "#guid" do
diff --git a/spec/lib/diaspora_federation/validators/person_validator_spec.rb b/spec/lib/diaspora_federation/validators/person_validator_spec.rb
index e4f7726..6b7d61d 100644
--- a/spec/lib/diaspora_federation/validators/person_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/person_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     it_behaves_like "a guid validator" do
diff --git a/spec/lib/diaspora_federation/validators/photo_validator_spec.rb b/spec/lib/diaspora_federation/validators/photo_validator_spec.rb
index 97682cc..342ebac 100644
--- a/spec/lib/diaspora_federation/validators/photo_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/photo_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     describe "#guid" do
diff --git a/spec/lib/diaspora_federation/validators/profile_validator_spec.rb b/spec/lib/diaspora_federation/validators/profile_validator_spec.rb
index d93b17a..5f8b52c 100644
--- a/spec/lib/diaspora_federation/validators/profile_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/profile_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { false }
     end
 
     %i[first_name last_name].each do |prop|
diff --git a/spec/lib/diaspora_federation/validators/related_entity_validator_spec.rb b/spec/lib/diaspora_federation/validators/related_entity_validator_spec.rb
index d42d3fc..56bf669 100644
--- a/spec/lib/diaspora_federation/validators/related_entity_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/related_entity_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     %i[local public].each do |prop|
diff --git a/spec/lib/diaspora_federation/validators/reshare_validator_spec.rb b/spec/lib/diaspora_federation/validators/reshare_validator_spec.rb
index e78905d..2f50ef5 100644
--- a/spec/lib/diaspora_federation/validators/reshare_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/reshare_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
     describe "#author" do
       it_behaves_like "a diaspora* ID validator" do
         let(:property) { :author }
-        let(:mandatory) { true }
       end
     end
 
@@ -23,9 +22,10 @@ module DiasporaFederation
     end
 
     describe "#root_author" do
-      it_behaves_like "a diaspora* ID validator" do
+      it_behaves_like "a property with a value validation/restriction" do
         let(:property) { :root_author }
-        let(:mandatory) { false }
+        let(:wrong_values) { ["i am a weird diaspora* ID @@@ ### 12345"] }
+        let(:correct_values) { [nil, "alice@example.org"] }
       end
     end
 
diff --git a/spec/lib/diaspora_federation/validators/retraction_validator_spec.rb b/spec/lib/diaspora_federation/validators/retraction_validator_spec.rb
index a0bb5c8..b19294c 100644
--- a/spec/lib/diaspora_federation/validators/retraction_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/retraction_validator_spec.rb
@@ -9,7 +9,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     describe "#target_type" do
diff --git a/spec/lib/diaspora_federation/validators/status_message_validator_spec.rb b/spec/lib/diaspora_federation/validators/status_message_validator_spec.rb
index 2fb5873..0f8bd4b 100644
--- a/spec/lib/diaspora_federation/validators/status_message_validator_spec.rb
+++ b/spec/lib/diaspora_federation/validators/status_message_validator_spec.rb
@@ -6,7 +6,6 @@ module DiasporaFederation
 
     it_behaves_like "a diaspora* ID validator" do
       let(:property) { :author }
-      let(:mandatory) { true }
     end
 
     it_behaves_like "a guid validator" do
diff --git a/spec/support/shared_validator_specs.rb b/spec/support/shared_validator_specs.rb
index 58a5c0a..16def0c 100644
--- a/spec/support/shared_validator_specs.rb
+++ b/spec/support/shared_validator_specs.rb
@@ -20,7 +20,6 @@ end
 shared_examples "a relayable validator" do
   it_behaves_like "a diaspora* ID validator" do
     let(:property) { :author }
-    let(:mandatory) { true }
   end
 
   describe "#guid" do
@@ -74,16 +73,18 @@ shared_examples "a diaspora* ID validator" do
     [nil, ""].each do |val|
       validator = described_class.new(entity_stub(entity, property => val))
 
-      if mandatory
-        expect(validator).not_to be_valid
-        expect(validator.errors).to include(property)
-      else
-        expect(validator).to be_valid
-        expect(validator.errors).to be_empty
-      end
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(property)
     end
   end
 
+  it "validates a well-formed diaspora* ID" do
+    validator = described_class.new(entity_stub(entity, property => "alice@example.org"))
+
+    expect(validator).to be_valid
+    expect(validator.errors).to be_empty
+  end
+
   it "must be a valid diaspora* ID" do
     validator = described_class.new(entity_stub(entity, property => "i am a weird diaspora* ID @@@ ### 12345"))