Envio de email dando erro no windows xp

Uso a seguinte função para envio de emails:

Public Shared Function enviaMensagemEmail(ByVal from As String, ByVal recepient As String, ByVal bcc As String, ByVal cc As String, ByVal subject As String, ByVal body As String, ByVal servidorSMTP As String, ByVal anexo As String)
Try
Dim mMailMessage As New MailMessage()

mMailMessage.From = New MailAddress(from)

mMailMessage.To.Add(New MailAddress(recepient))

If Not bcc Is Nothing And bcc <> String.Empty Then
mMailMessage.Bcc.Add(New MailAddress(bcc))
End If

If Not cc Is Nothing And cc <> String.Empty Then
mMailMessage.CC.Add(New MailAddress(cc))
End If

mMailMessage.Subject = subject

mMailMessage.Body = body

mMailMessage.IsBodyHtml = True

mMailMessage.Priority = MailPriority.High

If Not anexo Is Nothing Then
mMailMessage.Attachments.Add(New Attachment(anexo))
End If

Dim mSmtpClient As New SmtpClient(servidorSMTP)
Dim status As New System.Net.Mail.SmtpStatusCode

mSmtpClient.Credentials = New System.Net.NetworkCredential("email@fortaldesign.com.br", "senha", servidorSMTP)
mSmtpClient.Port = 25

mSmtpClient.Send(mMailMessage)

mMailMessage.Dispose()
mMailMessage = Nothing
mSmtpClient = Nothing
enviaMensagemEmail = True

Catch ex As Exception
MsgBox(ex.Message, MsgBoxStyle.Exclamation, "Erro!")
enviaMensagemEmail = False
End Try

End Function
Para enviar emails. Em minha máquina, que tem o VISTA, e em uma máquina virtual com o XP. Funciona normalmente, mas na máquina XP de meu cliente não. Apresenta o seguinte erro:

Syntax error in parameters or arguments. The server response was: 5.7.1 <webmaster@fortaldesign.com.br>...
Sender Ip must resolve.
E o restante do erro:

See the end of this message for details on invoking
just-in-time (JIT) debugging instead of this dialog box.

************ Exception Text ************
System.Net.Mail.SmtpException: Syntax error in parameters or arguments. The server response was: 5.7.1 <webmaster@fortaldesign.com.br>... Sender IP must resolve
at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at System.Net.Mail.SmtpClient.Send(String from, String recipients, String subject, String body)
at ControleRGI.email.enviaMensagemEmail(String from, String recepient, String bcc, String cc, String subject, String body, String servidorSMTP, String anexo)
at ControleRGI.backDatabase.envEmail_Click(Object sender, EventArgs e)
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

************ Loaded Assemblies ************
mscorlib
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///c:/WINDOWS/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll
----------------------------------------
ControleRGI
Assembly Version: 1.0.0.0
Win32 Version: 1.0.0.0
CodeBase: file:///C:/Arquivos%20de%20programas/FORTALDESIGN/Controle%20RGI/ControleRGI.exe
----------------------------------------
Microsoft.VisualBasic
Assembly Version: 8.0.0.0
Win32 Version: 8.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/Microsoft.VisualBasic/8.0.0.0__b03f5f7f11d50a3a/Microsoft.VisualBasic.dll
----------------------------------------
System
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
System.Windows.Forms
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System.Drawing
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
System.Runtime.Remoting
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Runtime.Remoting/2.0.0.0__b77a5c561934e089/System.Runtime.Remoting.dll
----------------------------------------
ADODB
Assembly Version: 7.0.3300.0
Win32 Version: 7.10.2346
CodeBase: file:///C:/WINDOWS/assembly/GAC/ADODB/7.0.3300.0__b03f5f7f11d50a3a/ADODB.dll
----------------------------------------
System.Xml
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Xml/2.0.0.0__b77a5c561934e089/System.Xml.dll
----------------------------------------
System.Configuration
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Configuration/2.0.0.0__b03f5f7f11d50a3a/System.Configuration.dll
----------------------------------------
System.Data
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_32/System.Data/2.0.0.0__b77a5c561934e089/System.Data.dll
----------------------------------------
CrystalDecisions.Windows.Forms
Assembly Version: 10.2.3600.0
Win32 Version: 10.2.51014.0
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/CrystalDecisions.Windows.Forms/10.2.3600.0__692fbea5521e1304/CrystalDecisions.Windows.Forms.dll
----------------------------------------
CrystalDecisions.Shared
Assembly Version: 10.2.3600.0
Win32 Version: 10.2.51014.0
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/CrystalDecisions.Shared/10.2.3600.0__692fbea5521e1304/CrystalDecisions.Shared.dll
----------------------------------------
CrystalDecisions.ReportSource
Assembly Version: 10.2.3600.0
Win32 Version: 10.2.51014.0
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/CrystalDecisions.ReportSource/10.2.3600.0__692fbea5521e1304/CrystalDecisions.ReportSource.dll
----------------------------------------
System.Web
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_32/System.Web/2.0.0.0__b03f5f7f11d50a3a/System.Web.dll
----------------------------------------
System.Transactions
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_32/System.Transactions/2.0.0.0__b77a5c561934e089/System.Transactions.dll
----------------------------------------
System.EnterpriseServices
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_32/System.EnterpriseServices/2.0.0.0__b03f5f7f11d50a3a/System.EnterpriseServices.dll
----------------------------------------

************ JIT Debugging ************
To enable just-in-time (JIT) debugging, the .config file for this
application or computer (machine.config) must have the
jitDebugging value set in the system.windows.forms section.
The application must also be compiled with debugging
enabled.

For example:

<configuration>
	<system.windows.forms jitDebugging="true" />
</configuration>

When JIT debugging is enabled, any unhandled exception
will be sent to the JIT debugger registered on the computer
rather than be handled by this dialog box.
Como posso corrigir o problema?