You maybe need to check if text string is empty , and if it isnt validate via regex
if(txtEmail1.Text.Equals(string.empty))
{
MessageBox.Show("Text is required");
}
else
{
Regex regex = new Regex(@"^([\w\.\-]+)@([\w\-]+)((\.(\w){2,3})+)$");
Match match = regex.Match(email);
if (match.Success)
{
}
else
{
MessageBox.Show("Email is not correct ");
}
}