The type of greeting used at the start of a business letter or email is important. Greetings are polite and necessary for formal writing. However, there are many instances in which you will not know the person who will read the letter. Therefore a greeting that is not gender specific but rather for an unknown person is acceptable.
"To whom it may concern" can be used for more formal letters, particularly for business letters. Other greetings that work for unknown genders include "Ladies and Gentlemen" or "Dear Sir or Madam." You can also substitute the person's position in the greeting instead of using a name, such as "Dear Vice President" or the company name.
It is possible to address a greeting to an unknown male as "Dear Sirs," "Dear Sir" or "Gentlemen." This is a more formal greeting but appropriate for letters dealing with any type of business. Unlike with women, you do not have different monikers based on marital status.
Greetings used when the addressee is female but unknown is similar to those greetings used for males. The two common greetings include "Dear Madam" or "Ladies." If you know the female's marital status, then several other greetings can be used. "Dear Mrs." is used for married woman, "Dear Miss" is used for unmarried women, though this is rarely used, and "Dear Ms." is used for unmarried women or women whose marital status is unknown.
Greetings for a group of people can use a collective noun. This way you do not have to list all the individuals in the group, which is useful if you do not know all the individuals in the group. Some formal greetings include "Dear Partners," "Dear Directors" or "Dear Customers." Other less formal greeting include "Hello, everyone," "Dear colleagues" or "Dear all."