The health insurance is a very good negotiating point for a formal separation agreement or divorce. I would not send that e-mail unless you are prepared to draw something up.
I don't see a problem with you asking him to reimburse you for his monthly share of the insurance. If he declines then I would say go ahead and drop him but still have a formal agreement drawn up.
It's too much money each month to keep paying without reimbursement for a spouse that is not in the marriage.
