Current CA law specifically says that electronic transmission of an offer and acceptance, without a pen and ink signature is binding. Back of a business card works too, as long as it meets all the other requirements of a contract. Amendments, addenda, modifications, the same thing. Conveyance electronically of an offer and acceptance of original contracts, and modifications of that contract are deemed to meet the “meeting of the minds” requirement. But the issues of signatures notwithstanding, to be binding, for real estate, it must be in writing. Phone calls don’t work.
