When writing to a company it is always Dear Sirs, unless you know that the company is owned/run by women, then it is Dear Mesdames. When writing to a firm it is the same, unless you are writing to a partner of the firm in which case you address it to that partner's name and then Dear Sir or Madam and then, when writing to anyone else in the firm, you address it to the firm with attention to whatever the person's name is, then Dear Sirs. When a woman chooses not to use Mrs Miss or Ms as a title, it is appropriate to use her actual name (first and last) although not in formal correspondence.