This is
sortof correct
You can send email as whoever you want that is true, but to verify this (as email servers do as part of the spam filter methods) you can link and validate it based on the originator properties (who actually sent it) based on the validation of the sender, this was recenty done when someone impersonated Larry Page (Google CEO) sending an email to Sergey Brin (Google cofounder) as they used a less secure key
http://www.businessinsider.com/a-mat...erview-2012-10
There are also validating the MX records for emails that are genuine but not from the originator (we use a 3rd party SMTP system that is validated via us creating MX records for the service's IP addresses)
I've noticed a lot of emails recently appearing to come from people I know (either facebook or follow on twitter) but the email addresses are obviously wrong and the content is junk