Can't speak to the legal standing of the e-mail in regards to the initiation of a contract, but from my understanding it sounds like you would be in the clear. Dealing with a partner who "isn't carrying his weight" is always a difficult topic, particularly when you are talking about a 2-person partnership. Hard to make a clear judgment based on the information provided, but your partner should understand the risks involved with investing and co-founding a start-up. Unless the wording of your agreement was overtly biased towards your partner, I find it hard to believe your partner would be able to re-coup his initial investment.