diff --git a/lib/email/sender.rb b/lib/email/sender.rb
index 087b8b8413b..6e58a704348 100644
--- a/lib/email/sender.rb
+++ b/lib/email/sender.rb
@@ -138,9 +138,9 @@ module Email
       when /\.mailjet\.com/
         @message.header['X-MJ-CustomID'] = @message.message_id
       when "smtp.mandrillapp.com"
-        @message.header['X-MC-Metadata'] = { message_id: @message.message_id }.to_json
+        merge_json_x_header('X-MC-Metadata', { message_id: @message.message_id })
       when "smtp.sparkpostmail.com"
-        @message.header['X-MSYS-API'] = { metadata: { message_id: @message.message_id } }.to_json
+        merge_json_x_header('X-MSYS-API', { metadata: { message_id: @message.message_id } })
       end
 
       # Suppress images from short emails
@@ -202,5 +202,13 @@ module Email
       )
     end
 
+    def merge_json_x_header(name, value)
+      mc_metadata   = JSON.parse(@message.header[name].to_s) rescue nil
+      mc_metadata ||= {}
+      mc_metadata.merge!(value)
+      @message.header[name] = nil
+      @message.header[name] = mc_metadata.to_json
+    end
+
   end
 end
diff --git a/spec/components/email/sender_spec.rb b/spec/components/email/sender_spec.rb
index 89d299dda4e..2f6f539f35d 100644
--- a/spec/components/email/sender_spec.rb
+++ b/spec/components/email/sender_spec.rb
@@ -139,6 +139,26 @@ describe Email::Sender do
       Then { expect(message.header['X-Discourse-Reply-Key']).not_to be_present }
     end
 
+    context "merges custom mandrill header" do
+      before do
+        ActionMailer::Base.smtp_settings[:address] = "smtp.mandrillapp.com"
+        message.header['X-MC-Metadata'] = { foo: "bar" }.to_json
+      end
+
+      When { email_sender.send }
+      Then { expect(message.header['X-MC-Metadata'].to_s).to match(message.message_id) }
+    end
+
+    context "merges custom sparkpost header" do
+      before do
+        ActionMailer::Base.smtp_settings[:address] = "smtp.sparkpostmail.com"
+        message.header['X-MSYS-API'] = { foo: "bar" }.to_json
+      end
+
+      When { email_sender.send }
+      Then { expect(message.header['X-MSYS-API'].to_s).to match(message.message_id) }
+    end
+
     context 'email logs' do
       let(:email_log) { EmailLog.last }