By Judy Vorfeld
According to The Gregg Reference Manual, Tenth Edition, by William A. Sabin, if you don’t know the names of anyone in a large organization, but know there are both men and women, use “Dear Sir or Madam” rather than “To Whom it May Concern.” (Section 1339)
Or, for an organization composed of men and women, use “Ladies and Gentlemen” or “Gentlemen and Ladies.”
Gregg has many guidelines, and one of them says (1338c) “Omit the salutation if you are using the simplified style, and replace it with the subject line.”
I would be tempted to do that rather than write “Dear Sir of Madam.” I suppose “Hey, Guys” just wouldn’t cut it.
Seriously, Section 1340s says you can also use the name of the organization in the salutation using “Dear” or “To.”
Here’s an excellent site: General Tips for Letter Writing
And to go to this very valuable site, click here. It tells about: Spoken and Written Forms of Address for U.S. Government Officials, Military Personnel, Foreign Officials, Nobility, and Religious Officials - correct forms of address for U.S. Public officials, diplomats, religious leaders, royalty, the British peerage, and military personnel. For each personage the chart gives the appropriate form or forms to be used in addressing letters, in letter salutations, in direct conversation, and in more formal introductions.
If you’re interested in addressing former office holders properly, click here.