What is the best way to addres a business letter to married couples?

I was taught that if you are writing a business letter to a married couple that the proper way to address it is, for example, "Mr. & Mrs. Joe Smith", using the husband's first name only. I have also been trained that even widows will continue to use the husband's name when the prefix "Mrs." is attached. It is important I get an expert's answer on this. No gut reactions or guesses, please. I need the correct professional business/formal answer. If you have a source you can name, I'd really appreciate it. Thank you

You are correct in your example. Unless they communicate otherwise, this is the proper way to address those who are married and widows. If someone prefers to be addressed differently then they will inform you, until then widows and married couples are to be addressed in the manner you described.

If there is a hyphenated name that the wife goes by then you address his name first, under which you address the wife's hyphenated name on the envelope. This would also be done in the opening of the letter.

The proper form of address for a married couple is "Mr. and(& works as well too) Mrs. John A. Smith(followed by either Esq. or II, III IV, Jr. etc.). For the widowed woman, "Mrs. John A. Smith" is completely acceptable as well. Only if the woman personally changes back to using her maiden name(i.e.-Mz. Jane Doe) or just "Mz." before her name(i.e.-Mz. John A. Smith).

according to etiqette expert,judith martin,also known in newspapers across the us as "miss manners",the correct form of adress,for example,john and mary smith,is as follows "mr. and mrs. john smith.