VB.Net Email Issue

Hello experts. I am having an issue with a very simple request form (vb.net) regarding sending an email once the submit button is pressed. My email server is exchange 2010. Here is the issue. Internally in my building the form works great. If I am working remotely I connect to my network using cisco vpn client. My issue is when connected via the cisco vpn client when I attempt to submit the form I get an error email not sent. I thought maybe it had something to do with my exchange server name so I placed my server IP address as well and still the same issue. My code is below if needed. So the question is why does this work internally but not remotely via vpn?

Public Function fnMailer(ByVal strPassEmailTo As String) As Boolean

        Dim strError As String = ""
        Dim MailMsg As MailMessage = New MailMessage

        MailMsg.Subject = "Service Request"
        MailMsg.Body = "Generated by system - Please do not reply to message" & vbCrLf & vbCrLf

        MailMsg.From = New MailAddress("test@test.com")

        MailMsg.To.Add(strPassEmailTo)

        MailMsg.Body += Global_mailMessage
        Dim mailClient As New System.Net.Mail.SmtpClient("servername")

        mailClient.UseDefaultCredentials = True

        Try
            mailClient.Send(MailMsg)
        Catch ex As SmtpException
            MessageBox.Show("Error sending email")
            Return False
        End Try

        Return True

    End Function