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;
}
}