C# clean up email

CleanUpEmail.cs

public static string CleanUpEmail(string emailString)
{
    emailString = emailString.ToLower();
    string normalized = emailString.Normalize(NormalizationForm.FormD);
    StringBuilder resultBuilder = new StringBuilder();

    foreach (var character in normalized)
    {
        UnicodeCategory category = CharUnicodeInfo.GetUnicodeCategory(character);

        // Strip all characters except uper/lower case letters 
        if (category == UnicodeCategory.LowercaseLetter
            || category == UnicodeCategory.UppercaseLetter
            || character == '_'
            || character == '.'
            || character == '@')
        {
            resultBuilder.Append(character);
        }
    }
    return resultBuilder.ToString();
}