Terry Zink posts about the biggest problem with security: human errors. Everyone who is looking at security needs to think about the human factor. And how people can deliberately or accidentally subvert security.
You can't technical your way out of the bulk folder. I wrote that a year and a half ago, and it's even more true today. Filters at the big webmail providers continue to evolve to meet new threats and new spamming techniques. Sending technically perfect mail won't get your mail into the inbox. Recipients have to want the mail and interact with the mail for good delivery.