Is a husband's waiver of his share in the house to his wife via an electronic message sufficient, Islamically and customarily, for it to be her right, and can he reclaim it?

What your husband wrote for you is considered a gift (hibah (definition)), but he has the right to retract it. This is because, according to the more preponderant opinion, a gift does not become binding until it is taken possession of (qabd). Before possession and acquisition, the gift remains the property of the donor, who can dispose of it as he wishes. So, if he did not leave you to deal with his share in the house and did not put you in possession of it, he has the right to retract it. "Possession" or "acquisition" means that you dispose of the gifted item and the donor relinquishes his control over it.