Speaking as a board member who was on a board that got an email like yours, but with very different circumstances (I've mentioned my situation where one person ran everything, and basically, she quit when she didn't get something very specific she wanted, but which the board felt was not in the best interest of the organization in the long-run), the board is going nuts right now. Emails and phone calls are flying, and they are busy trying to figure out who will speak to you and what to say; who will speak to Dee and what to say; and yes, they are trying to figure out how to replace you because they know you may be gone and they still have a major project to deal with. There are going to be some angry board members - you left, for good reason, but at a bad time, and they will be upset about that, so the emails are going to be flying. If you don't hear from them in about a week, just wash your hands of them and walk away knowing you are better without them in your life. I do think you'd be correct in contacting your friends and letting them know you've resigned from the organization, but would still like to be friends with them.