One book suggests: The Rev. and Mrs. Tom R. Anderson on the envelope. Start a business letter with Reverend Sir: Start a social letter with My dear Mr. Anderson:
Yes, it is old-fashioned. Perhaps that's what we need to combat the incivility among so many younger and older people nowadays. Nevertheless, I am eager to know what the "modern" way is. Please give us an example so that the questioner , who I assume is a younger person, can use it as a model. Thank you.
Many, if not all, married women today prefer not to be spoken of as eg 'Mrs. Tom Anderson'. They feel it diminishes their individuality and their equality with their husband.
At the start of a business letter and of a social letter Dear Reverend Smith.