Well it depends, if you are Muslim you sholud say "es-selamu-alejkum" ("may peace be with you"), and he would reply "alejku mu selam", but if you aren't Muslim you can say any regular respectful greeting.
Desmond Tutu was a Bishop, so the correct salutation would be, "Your Excellency".
the correct salutation for two men is Messrs
"Dr and Mrs Smith" is the correct salutation. (With Smith replaced with the correct surname)
After the salutation of a business letter, the correct punctuation to use is a colon.
Dear ms
madame mayor
Mrs. or Ms.
His Excellency, Ambassador of ................... This salutation can be used when writing to the Forein ambassadors of all countries.
chief counsel
Dear Sirs and Madam:
Consul of China
Honorable Consulate