Since writing this Salesforce has released support for DKIM which is much better way of authenticating emails coming out of Salesforce which I would now use if your infrastructure allows it. Check out the release notes for more information.
@radnip what do you mean that email isn’t secure. Damn!
— Simon Goodyear (@simongoodyear) May 26, 2015
Yes Simon and his brilliant sarcastic wit 🙂 But I was talking to company that needed a copy of my passport as ID. They asked if I could email it to them as an image. Now around 40-50% of all emails going around the internet is not encrypted. Anyone sitting between my email server and the destination server would be able to read my email if not secured… (you can check if an email service supports encryption here).