Questions re Formal Address on Envelope, Personalized Letter

We are sending Thank You letters to the benefactors and volunteers of our fundraising event. The letter envelope is addressed to Mr. and Mrs. (John Smith, for example). The letter salutation, however, will be less formal because we know them, personally. Is there a rule as to whose name should be listed first, John's or his wife's?

Even if there is such a rule, if John's wife was the volunteer, shouldn't her first name be listed before his in the saluation?

We appreciate any assistance you can offer.

~Mary