I think the only people I'd address as "Dear Mr Smith" would be customers/members of the public, or perhaps very, very senior people in a large company. Colleagues, clients, contractors and so forth would be "Dear Jane", even if I'd never met them before. But then, first name terms are pretty much universal in every organisation I've ever worked for here.


These days, it seems pretty much universal to start business emails with "Hi John", although "John -" would be an option too, especially for short messages. Some people don't bother with a salutation at all, but that seems kind of abrupt to me.