I would guess "Monsieur le Maire," without Cher. and "Veuillez recevoir, Monsieur le Maire, l'assurance de mes sentiments les plus distingués". But don't take my word for it! You might want to ask in http://forum.wordreference.com/forumdisplay.php?f=3 as there are quite a few native French speakers there who will usually give a quick answer. I'm unsure about email-writing as opposed to letter-writing etiquette, both in French and in English!