After sending a ticket Jodohost kindly responded with a test script which worked fine.
Further investigation of that test script then showed that Jodohost do not appear to be fully supporting System.Net.Mail which I was using. When I switched to System.Web.Mail (which was shown in the JodoHost test script) everything worked fine.
Thus anyone else who has experienced problems with email being sent via their web site might want to check this.
The script that works is based upon the following - this comes from the JodoHost test script:
Sub btn_Click(sender as Object, e as System.EventArgs)
If Request.Form("Email") <> "" Then
Dim objMail As New MailMessage()
objMail.From = "[email protected]"
objMail.To = Request.Form("Email")
objMail.Subject = Request.Form("Subject")
objMail.Body = Request.Form("Message")
objMail.BodyFormat = MailFormat.Text
SmtpMail.SmtpServer = "localhost"
SmtpMail.Send(objMail)
Else
lblResponse.Text = "Please enter an email address."
End If
End Sub