Type this code on the textBox leave event...

System.Text.RegularExpressions.Regex rEMail =

new System.Text.RegularExpressions.Regex

(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");

if (textBox3.Text.Length > 0)

{

if (!rEMail.IsMatch(textBox3.Text))

{

MessageBox.Show("E-Mail expected", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

textBox3.SelectAll(); e.Cancel = true;

}

}