I'd agree on what Anglika said. Not once have I received a letter with a colon following 'Dear'.
We've all been taught to use a comma, and now, all of a sudden(to me, at least), we have a differentiation (AmE, BrE).
Seemingly flawless usually gets full of flaws by inquiring closely one state which was due to the moment of observing, so do as you think you should...