Check PA corporate law for 'director' resignation.
'Most' corporate law requires resignations to be submitted in writing and SIGNED.
95% sure PA law will require a signed letter of resignation.
http://www.legis.state.pa.us/WU01/LI/LI/CT/HTM/15/15.HTM
1724. Term of office of directors.
(a) General rule.--Each director of a business corporation shall hold office until the expiration of the term for which he was selected and until his successor has been selected and qualified or until his earlier death, resignation or removal.
Any director may resign at any time upon written notice to the corporation. The resignation shall be effective upon receipt thereof by the corporation or at such subsequent time as shall be specified in the notice of resignation. Each director shall be selected for the term of office provided in the bylaws, which shall be one year and until his successor has been selected and qualified or until his earlier death, resignation or removal, unless the board is classified as provided by subsection (b). A decrease in the number of directors shall not have the effect of shortening the term of any incumbent director.
5724. Term of office of directors.
(a) General rule.--Each director of a nonprofit corporation shall hold office until the expiration of the term for which the director was selected and until a successor has been selected and qualified or until the director's earlier death, resignation or removal. Directors, other than those selected by virtue of their office or former office in the corporation or in any other entity or organization, shall be selected for the term of office provided in the bylaws. In the absence of a provision fixing the term, it shall be one year.
(b) Resignations.-
-Any director may resign at any time upon notice in record form to the corporation. The resignation shall be effective upon its receipt by the corporation or at a subsequent time specified in the notice of resignation.
What does your Corporate Attorney say ?
Not getting a DEFINATIVE answer for a legal question from a legal professional would, IMO, constitute nonfeasance.