A simple rule for validating an observable which could contain multiple email addresses. Assuming you have an observable like self.emailTO = ko.observable('');, you would just extend it to have self.emailTO.extend({ multiemail: true });. Use it with the required validator if you want at least one email address. ko.validation.rules['multiemail'] = { validator: function (val, validate) { if (!validate) { return true; } var isValid = true; if (!ko.validation.utils.isEmptyVal(val)) { // use the required: true property if you don't want to accept empty values var values = val.