EmailValidator.cs

public class EmailValidator : Behavior<Entry>
    {
        const string emailRegex = @"^(?("")("".+?(?<!\\)""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" +
        @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9]))$";

        public static readonly BindableProperty IsValidProperty = BindableProperty.Create("IsValid", typeof(bool), typeof(EmailValidator), false);

        public bool IsValid
        {
            get { return (bool)base.GetValue(IsValidProperty); }
            private set { base.SetValue(IsValidProperty, value); }
        }

        protected override void OnAttachedTo(Entry entry)
        {
            entry.TextChanged += HandleTextChanged;
        }

        void HandleTextChanged(object sender, TextChangedEventArgs e)
        {
            IsValid = (Regex.IsMatch(e.NewTextValue, emailRegex, RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(250)));
            ((Entry)sender).TextColor = IsValid ? Color.Default : Color.Red;
        }

        protected override void OnDetachingFrom(Entry entry)
        {
            entry.TextChanged -= HandleTextChanged;
        }
    }