As general rule, it's perfectly acceptable to use Miss or Mrs. if you know that the lady in question is either of those two. However, if you don't know anything about the lady to whom you're writing then using Ms makes sure you don't offend her.I don't think that such considerations are silly, Anglika. Some women resent being categorized either as unmarried or married, as if they derive their identities from having caught a man.
If a woman wants to stick with Mrs or Miss that's fine too. I'm sure that they will inform you. Ms is simply a neutral term and as such it works well.
I never said that you should not consider the feelings of the person being addressed.
What I said was that there is no law of privacy requiring the use only of "Ms" when addressing a letter to a woman. The very idea is completely ludicrous.