asp.net (VB) This mail server requires authentication when attempting to send to a non-local e-mail address

Hello, All.
I am trying to get a contact email script to work.
It works when I send it to a local user on the same domain.
However, I get this when I try sending it to a user on another domain.

System.Net.Mail.SmtpException: 'Bad sequence of commands. The server response was: This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.'

This is my code.

Front end

    <form id="form1" runat="server">
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <td>From:</td>
            <td><asp:TextBox ID="txtTo" runat="server" /></td>
        </tr>
        <tr>
            <td>Subject:</td>
            <td><asp:TextBox ID="txtSubject" runat="server" /></td>
        </tr>
        <tr>
            <td valign = "top">Body:</td>
            <td><asp:TextBox ID="txtMessage" runat="server" TextMode="MultiLine" Height="150" Width="200" /></td>
        </tr>
        <tr>
            <td></td>
            <td><asp:Button ID="btnSend" Text="Send" runat="server" OnClick = "SendEmail" /></td>
        </tr>
    </table>
    </form>

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Back-end

Imports System.Net.Mail
Partial Class VB
    Inherits Page

    Protected Sub SendEmail(sender As Object, e As EventArgs) Handles btnSend.Click
        Try
        Dim mail As New MailMessage
        Dim smtpServer As New SmtpClient("mail.yourdomain.com")
        Dim strSender As String = "info@yourdomain.com"
        Dim txtPass As String = "password"

        mail.From = New MailAddress(strSender)
        mail.To.Add(txtTo.Text)
        mail.Bcc.Add(strSender)
        mail.Subject = txtSubject.Text
        mail.Body = txtMessage.Text

        smtpServer.Port = 25
        Dim credential As Net.NetworkCredential = New Net.NetworkCredential(strSender, txtPass)
        smtpServer.Credentials = credential
        smtpServer.EnableSsl = False
        smtpServer.Send(mail)

        Response.Write("Your email has been sent, maybe?")
        smtpServer.Dispose()
        Catch ex As Exception
        MsgBox(ex.Message, vbCritical)
        End Try
    End Sub
End Class

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I've been working with cdosys for well over a decade now with classic asp, so I am in uncharted territory with this one.

Any help on this would be great.
Wayne