1
general::
To the appropriate recipient for this message, as in I didn't know who was responsible for these complaints so I just addressed it "to whom it may concern." This phrase is a formula used in letters, testimonials, and the like when one does not know the name of the proper person to address. [Second half of 1800s]
American Heritage Idioms
2
general::
Phrase(s): to whom it may concern
Cliché to the person to whom this applies. (A form of address used when you do not know the name of the person who handles the kind of business you are writing about.) • The letter started out, "To whom it may concern." • When you don’t know who to write to, just say, "To whom it may concern."
McGrawhill's American Idioms And Phrasal Verbs