How do I address a letter to a board of directors?

Traditionally, a board of directors is addressed via the chair, by name. As such, you could address the letter:

Ms. Maria Manundo,

Chair, Board of Directors

Agency X

and then address the letter:

Dear Ms. Manundo,

I write to you as the chair of the board of agency x to ..... (fill in the reason for your letter)

Alternatively, you could address the letter to the entire board. As in:

Board of Directors

Agency X

Dear Board,