E-mail is like any other form of communication. The first and foremost rule is: consider your audience. As you describe, random neural dumps are less than useful. If you don't consider your audience, you can damage your credibility. As an engineer who gets to write a fair amount of documentation, some as e-mail, I write very differently for customers, sales/marketing, and other engineers.

Regarding signature blocks: unfortunately some of us work in corporate environments that mandate signature block contents and format. Given a choice I would use my full name and snail mail address for external e-mail. Sadly, my company-enforced sig outweighs a lot of my message contents.