Recently I was proof reading an email that a coworker was going to send to a superior and I pointed out that the valediction wasn't capitalized. He said it didn't need to be because it was the start of a phrase or clause and not a sentence, and that people only do it because other people do it. But in the back recesses of my mind I'm thinking that there is a rule that says that it should be capitalized.
Is my coworker right and capitalization is unnecessary in this case? If I'm correct, what rule or guide can I point to, to prove my stance?