First, there is not such thing as an "involuntary" resignation: involuntarily losing your job is being fired or terminated or laid off. If you resign, you are doing so voluntarily--it is your choice, since while your manager can try to browbeat you or convince you to resign, he *cannot* make you resign against your bill. Bear in mind that if you do to resign, you will make yourself ineligible for unemployment benefits.The law does not offer any advice or rules or guidance on whom to send a resignation letter to, if you choose to send such a letter: send it to whatever manager or executive you think best, based on your personal knowledge of them.