Validating user input often includes validating whether the user have supplied a valid email address - validating email addresses often take the course of regular expressions, however there seems to be no definite solution. I once saw a guy who just tested whether the .Net Framework MailAddress would instantitate with the address and I thought it was great so since then that is what I have been doing (the method of course is not so great if you want to validate client side).
bool emailAddressIsValid = Utils.Email.ValidateAddress("rasmus@webmodelling.com");
public static bool ValidateAddress(string pEmailAddress) { bool addressIsValid = true; try { MailAddress mailAddress = new MailAddress(pEmailAddress); } catch { addressIsValid = false; } return addressIsValid; }