Mix,open,close punctuation

Open punctuation has no punctuation after the salutation or complimentary closing, no commas in the addressee's address and no periods after abbreviations:

The President of the United States
Washington DC

Dear Mr Bush

Sincerely yours


Closed punctuation has punctuation after the salutation and closing, in the address and after abbreviations

The President of the United States,
Washington, D.C.

Dear Mr. Bush:

Sincerely yours,


I suppose that 'mixed' punctuation is a middle way between these, and I also suppose that there is considerable difference of opinion as to whether and which punctuations are used where.