Emailing from VB.NET APP

Hi everyone.

I have a problem sending emails from my vb.net program. It is fine sending to email address's on our domain, but doesn't work if I try and send to an external address e.g. hotmail account. No error is reported, the email is just never recieved.

Code is as follows:

Dim Message As New System.Web.Mail.MailMessage
With Message
    .To = "<myexternaladdress@hotmail.co.uk>"
    .From = "<myapp@mydomain.co.uk>"
     .Body = "My test mail"
     .Subject = "My subject"
End With

Dim MailServer As System.Web.Mail.SmtpMail
MailServer.SmtpServer = "555.555.555.5"   'IP of my exchange server (version 5.5 sp2)
MailServer.Send(Message)

Any help would be great

Thanks