Hang on everyone. We are in a transition period. In life everyone gets to choose their name & be called by it. This is the modern way and quite right too.
However we have old rules for addressing things which are based on etiquette and correspondence conventions. These have nothing to do with what you think of the person and everything to do with signalling you know the rules.
Like - if you know the person's name; sign the letter with yours sincerely. If you don't, sign yours faithfully.
Part of those rules include - if you address a letter you put Title of man then man's initial then man's surname. If you write to his wife you put Title of woman, husband's initial, husband's surname.
I think the old conventions, obviously, are not fit for purpose now and have an incorrect ring to them. Modern usage is to put first name/ surname of every person- or some posters here have suggested just first names, or the informal all at number 47, etc.
Posters like Salem are stubbornly clinging to the old way because they clearly love the unprogressive social order it implies!
But other posters, like the lady who said she knew someone had kept her name but still addressed it to Mr. A. Husbandname, asked "is that wrong?" She really didn't know.
Clearly we have a clash of styles as change is in progress. The people writing Mrs DHinitial Husbandname are sometimes making a sarky point, but some are adhering to what they consider is correct.
Different definitions of correctness and respect going on here. So don't be quick to judge motives.