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You have to stand up for yourself. Not to the point where it becomes a power struggle but enough to let her know that you won't take the intimidation of these constant emails. That many emails is insane. Unfortunately if she is sending these emails she is entrenched in being the go between in your relationship. You have to address this. You have to be honest and firm in responding to this problem. Let her know that you will hear her opinion but then you are standing firm in taking up for yourself. Tell her to back off but be respectful.
As far as your fiancee he is the one allowing this dysfunction. It isn't natural to have one's mother so into a relationship when you are an adult. You have to work on this as a couple even if you have counseling because this will ruin the relationship. Sooner or later you are going to feel that he is choosing mom.
Look into some short term counseling
It is a very delicate situation. Mother has this role in his life that is much longer than yours. You may have to show him that there needs to be compromise over a long period of time. He may not accept it right away but it is possible to get him to come around. There is no other way to resolve this then to get him on board.