Referring to a woman in the US as "Mrs. Peter Smith" is traditionally correct here for formal written correspondence. The rule always was, once her husband dies or she divorces then she's "Mrs. Mary Smith."
The spoken address form of "Mrs. Peter Smith" was much more prevalent in the Southern US than elsewhere. I remember some years back when a very prominent lady in South Carolina society, Mrs. Henry Reynolds, was referred to by nearly everyone - even when speaking directly to her - as "Miz Henry." I don't recall her first name, but presumably only her family and VERY close friends might have dared to use it.
What are the more "liberated" forms of address now in Germany? Do you use anything for women that doesn't denote a mariage status, like "Ms." is used over here?