Allow empty string for EmailAddressAttribute

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private string _Email;
[EmailAddress(ErrorMessage = "Ingrese un formato de email válido")]
public string Email { get { return _Email; } set { _Email = string.IsNullOrWhiteSpace(value) ? null : value; } }

I used the suggestion from Yishai Galatzer to make a new ValidationAttribute called EmailAddressThatAllowsBlanks:

namespace System.ComponentModel.DataAnnotations
{
    public class EmailAddressThatAllowsBlanks : ValidationAttribute
    {
        public const string DefaultErrorMessage = "{0} must be a valid email address";
        private EmailAddressAttribute _validator = new EmailAddressAttribute();

        public EmailAddressThatAllowsBlanks() : base(DefaultErrorMessage)
        {

        }

        public override bool IsValid(object value)
        {
            if (string.IsNullOrEmpty(value.ToString()))
                return true;

            return _validator.IsValid(value.ToString());
        }
    }
}

You have two options:

1. Convert string.Empty to null on the Email field. Many times that is perfectly acceptable. You can make this work globally, or by simply making your setter convert string.Empty to null on the email field.
2. Write a custom EmailAddress attribute, since EmailAddressAttribute is sealed you can wrap it and write your own forwarding IsValid method.

Sample:

bool IsValid(object value)
{
    if (value == string.Empty)
    {
        return true;
    }
    else
    {
        return _wrappedAttribute.IsValid(value);
    }
}

Expansion on option 1 (from Web Api not converting json empty strings values to null)

Add this converter:

public class EmptyToNullConverter : JsonConverter
{
    private JsonSerializer _stringSerializer = new JsonSerializer();

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(string);
    }

    public override object ReadJson(JsonReader reader, Type objectType,
                                    object existingValue, JsonSerializer serializer)
    {
        string value = _stringSerializer.Deserialize<string>(reader);

        if (string.IsNullOrEmpty(value))
        {
            value = null;
        }

        return value;
    }

    public override void WriteJson(JsonWriter writer, object value, 
                                   JsonSerializer serializer)
    {
        _stringSerializer.Serialize(writer, value);
    }
}

and use on the property:

[JsonConverter(typeof(EmptyToNullConverter))]
public string EmailAddress {get; set; }

or globally in WebApiConfig.cs:

config.Formatters.JsonFormatter.SerializerSettings.Converters.Add(
    new EmptyToNullConverter());