Use #parts instead of #part to read all the parts of the Message.

Looks like I was accidentally using #part which was adding an extra
empty Mime section

lib/email_template_interceptor.rb

@@ -5,8 +5,8 @@ class EmailTemplateInterceptor
   def self.delivering_email(message)
     # Remove HTML part if HTML emails are disabled.
     unless current_application_settings.html_emails_enabled
-      message.part.delete_if do |part|
-        part.content_type.try(:start_with?, 'text/html')
+      message.parts.delete_if do |part|
+        part.content_type.start_with?('text/html')
       end
     end
   end