Email Sending Problem in .Net

I am using asp.net for sending mail.Code is:-

Dim message As New MailMessage()

message.To.Add(New MailAddress(abc@yahoo.com))

message.From = New MailAddress(xyz@yahoo.com)

 message.Body = txtMessage.Text

message.Subject = txtSubject.Text.Trim()

Dim attach As Attachment = New Attachment(txtAttache.Text)

message.Attachments.Add(attach)

message.Body = txtMessage.Text

message.IsBodyHtml = True

Dim client As New SmtpClient("127.0.0.1") //Server IP

client.Send(message) MessageBox.Show("MAIL SENT SUCCESSFULLY")

 It works fine. But if some-one uses Reliance Data-Card in laptop OR in home pc or other service likefrom AIRTEL or BSNL for internet connection in home pc.

Then what is the IP that I have to use in SMTP client

Dim client As New SmtpClient( ...