'Dear Mayor Smith' is acceptable for ordinary communication. 'Your Honor' is a more formal salutation. In Canada (and, I think, in Britain also) it might be better to use 'Your Worship.' (A mayor historically had the powers of a magistrate.) Check with the municipality's office of protocol when in doubt.
Mr. Mayor
Salutations are gestures or greetings that are meant as an acknowledgment. The proper salutation for a current mayor is Mr. Mayor or Madam Mayor.
"Your Honor" or "Mr. (Mrs.) Mayor"
Dear Mr. Mayor or sometimes, such as New York, Your Honor.
The proper salutation for a Mayor and her husband would typically be "Mayor [Last Name] and Mr. [Husband's Last Name]." If the husband holds a title or position, it may also be appropriate to acknowledge that title. In written communications, you might start with "Dear Mayor [Last Name] and Mr. [Husband's Last Name]." Always consider the preferences of the individuals when addressing them.
madame mayor
The mayor is generally referred to as Sir, or Mister Mayor. His wife, unless she holds a political office herself, is called Mrs. Whatever-her-last-name-is. - wjs1632 -
Mr. John Jones.
The proper salutation for a probate judge is "The Honorable [Judge's Full Name]."
What is the proper salutation for a register of wills, e.g. Honorable John White?
The proper salutation when writing to a judge is "Dear Judge [Last Name]" or "Your Honor."
the correct salutation for two men is Messrs