'Mr.' is used for any man.
'Mrs.' is used for a married woman who has taken her husband's last name.
'Ms.' may be used for any woman, married or not.
If you don't know the person's gender, you can address your correspondence to Mr./Ms. Xxxxx Xxxxx.
Beginning a message, you can use the formula of
'Dear Sir or Madam:
I hope this is helpful,
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