The Terms of Your License. Your license sounds like it had only one condition: anonymity. Since the poster has now breached that condition, you have a right to revoke your license -- that is, have the email removed. If the poster is disregarding your request, you have to try a more forceful strategy. One approach may be to contact the other party's service provider and send a DMCA notice. (This book -- A Legal Guide to Web & Software Development -- explains how to do it.) Another possibility is to hire an attorney to threaten the poster with a lawsuit claiming copyright infringement and invasion of privacy.
Is It Worth the Effort? Keep in mind that hiring a lawyer or sending a DMCA notice should really only be used if the posted email is causing you some harm. These are not sure-fire strategies -- for example, the poster may assert valid copyright defenses -- and they prove expensive. The Dear Rich Staff realizes this may not be what you wanted to hear ... but sometimes the easiest approach may be to let go and move on.