Hi vereato,

Check this example. Now please take its reference and correct your code.

HTML

<asp:TextBox ID="txtEmail" runat="server" />
<asp:Label ID="lblMessage" runat="server" ForeColor="Red" Visible="false"></asp:Label>
<br />
<br />
<asp:Button Text="Submit" runat="server" OnClick="OnSubmit" />

Code

C#

protected void OnSubmit(object sender, EventArgs e)
{
    lblMessage.Visible = false;
    lblMessage.Text = "";
    if (!string.IsNullOrEmpty(txtEmail.Text.Trim()))
    {
        System.Text.RegularExpressions.Regex regex = new System.Text.RegularExpressions.Regex(@"^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$");
        bool isValid = regex.IsMatch(txtEmail.Text.Trim());
        if (!isValid)
        {
            lblMessage.Visible = true;
            lblMessage.Text = "Invalid email address";
        }
    }
    else
    {
        lblMessage.Visible = true;
        lblMessage.Text = "Required";
    }
}

VB.Net

Protected Sub OnSubmit(ByVal sender As Object, ByVal e As EventArgs)
    lblMessage.Visible = False
    lblMessage.Text = ""
    If Not String.IsNullOrEmpty(txtEmail.Text.Trim()) Then
        Dim regex As Regex = New Regex("^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$")
        Dim isValid As Boolean = regex.IsMatch(txtEmail.Text.Trim())

        If Not isValid Then
            lblMessage.Visible = True
            lblMessage.Text = "Invalid email address"
        End If
    Else
        lblMessage.Visible = True
        lblMessage.Text = "Required"
    End If
End Sub

Screenshot