There are many ways to validate if an e-mail address is right, and many wrong ways.  The biggest wrong way is using a Regular expression, why?  They are complex and there is not one that resolves this issue, you will always get an invalid e-mail address that passes through okay.

So what can we do?  Why not use something that already works, something that someone else maintains, something we already have?

Let’s take a look at the System.Net.Mail name space, it has MailAddress, and if you add a string that is an invalid e-mail address it fails eg:

new MailAddress("invalidemailaddress");

This causes an exception to be thrown.

This mean you can use a single line of code to see if an e-mail is valid or not.

Let’s make it more practical and generate a ValidationAttribute that validates an email address for any given field in a model, what we are after doing is something like this:

    public class UserModel
    {
        [Required]
        public string name { get; set; }
        
        [Email]
        public string email { get; set; }
    }

To does this we just need to create an email attribute:

public class EmailAttribute : ValidationAttribute
    {
        protected override ValidationResult IsValid
        (object value, ValidationContext validationContext)
        {
            if (string.IsNullOrWhiteSpace(value?.ToString())) return ValidationResult.Success;

            try
            {
                new MailAddress(value.ToString());
            }
            catch (Exception)
            {
                return new ValidationResult("Please provide a valid email address.");
            }
            return ValidationResult.Success; 
         } 
     }

So simple when you know how to do it.