Clergy
ENVELOPE: "The Rev [Edward] ...."
OPEN: "Dear Mr [Smith]" or "Dear Father [Smith]"
CLOSE: "Yours sincerely"

Countess
ENVELOPE: "The Rt Hon the Countess of .....," or socially "The Countess of ....."
OPEN: "Madam" or socially "Dear Lady ..."
CLOSE: "Yours faithfully" or "Yours truly"

Dame
ENVELOPE: "Dame [Sarah Smith]" followed by the initials of the appropriate Order eg "GCB".
OPEN: "Madam" or socially, "Dear Dame [Sarah Smith]" or "Dear Dame [Sarah],"
CLOSE: "Yours faithfully"

Dean
ENVELOPE: "The Very Rev the Dean of ....."
OPEN: "Dear Dean"
CLOSE: "Yours sincerely"

Duchess
ENVELOPE: "Her Grace the Duchess of ...." or socially "The Duchess of ..."
OPEN: "Madam", or socially, "Dear Duchess of ...."
CLOSE: "Yours faithfully"

Duke
ENVELOPE: "His Grace the Duke of ...." or socially "The Duke of ...."
OPEN: "My Lord Duke, " or "Dear Duke"
CLOSE: "Yours faithfully"

Earl
ENVELOPE: The Right Hon the Earl of ..." or socially " The Earl of ...."
OPEN: "My Lord" or socially , "Dear Lord"
CLOSE: "Yours faithfully"

Earl's Wife
ENVELOPE: "The Rt Hon the Countess of .....," or socially "The Countess of ....."
OPEN: "Madam" or socially "Dear Lady ..."
CLOSE: "Yours faithfully" or "Yours truly"

High Commissioner
ENVELOPE: "His Excellency Mr. ...."
OPEN: "Dear High Commissioner.."
CLOSE: "Yours sincerely"