How to address letters in the modern world

Doodle, not off track at all, as far as I am concerned.. I have that very thought every time I have to write to or email some unknown person! ☺

If I have to write, my principle is to observe the old rules and then I can't go wrong. My only quandary, when addressing Christmas cards is how to address a couple both or whom have doctorates. I usually just use x* and Y* Z*******. Both forenames and surname, never mind the doctorates.

Sorry a bit off track but what is the correct form these days for a business letter now that Dear Sir/Madam is a no no?

Obvious, dispense with written communication in any firm.
An email or text starting “Hi guys” surely?

For many years, for anyone of my own generation, I have just put John and Janet Jones, or Janet and John Jones, if I know Janet better.
For older relatives like my parents, I'd usually use the trad. Mr and Mrs J Jones , since I knew they preferred it.
I'd never use it for anyone of my own generation though.

For professional reasons a sister and a dd kept their own surnames, so in such cases I'd put Janet Green and John Smith - or just Janet and John.

I send Christmas and Easter cards to a never-married lady of 80 odd who was my mother's cleaning lady for many years. I do address them to Miss Janet Jones - I don't think she'd like to be a Ms.

Actually, Monica, your last paragraph probably applies to me too ?

I generally follow the old rules on Mr & Mrs, and a single person. For those living together and married but with different surnames then Jane Smith & John Brown, why I do not put Ms J Smith & Mr J Brown, I am not sure.

Thinking about it the honest answer is that it depends on the moment, the name and whether I am feeling slapdash or not, and actually I generally disregard the rules.