FIX: always set a default Message ID in email

lib/email/sender.rb

@@ -70,6 +70,10 @@ module Email
       post_id = header_value('X-Discourse-Post-Id')
       reply_key = header_value('X-Discourse-Reply-Key')
 
+      # always set a default Message ID from the host
+      uuid = SecureRandom.uuid
+      @message.header['Message-ID'] = "<#{uuid}@#{host}>"
+
       if topic_id.present?
         email_log.topic_id = topic_id
 

spec/components/email/sender_spec.rb

@@ -73,6 +73,11 @@ describe Email::Sender do
       Then { expect(message.header['List-ID']).to be_present }
     end
 
+    context "adds a Message-ID header even when topic id is not present" do
+      When { email_sender.send }
+      Then { expect(message.header['Message-ID']).to be_present }
+    end
+
     context "adds Precedence header" do
       before do
         message.header['X-Discourse-Topic-Id'] = 5577