Companies seem to ignore the single largest online branding/advertising venue available: their own regular external emails. Why not use these emails to market the senders company?

You have a website.
You send emails.

Why not multiply your sales-staff by “wrapping” the regular email in an interactive letterhead?

No other marketing or advertising medium is as targeted as an email between people that know each other (as opposed to mass emails). These emails are always read and typically kept.