I tend to find that most people who say "I've GOT to be brutally honest with people, it's just the way I am!" are really using that as an excuse to cover for their rudeness and the fact that they enjoy being jerks who yank other people's chains.
Anyone with an ounce of internet savvy and sense knows that once you hit that "send" button, it's out of your hands, and your control. Any expectation of "privacy" and "confidentiality" is naive at best, stupid at worst and is nothing but a delusion on the sender's part. Accordingly, once anything arrives in a mailbox, either snail or electronic, it becomes the property of the recipient, to do with as they see fit. That's why it behooves anyone to think twice, nay three times, then store it under drafts and think again, before hitting that "send" button, especially something highly and personally critical of the recipient.
I truly doubt "Mr" Jones has the 'nads to walk up to the Smith trailer, in person, and say in front of everyone the same type of drivel that he sent to Matt under the cover of "Private" messaging.