Again Email sending error

Hi every one

I have one application have multiple users I need to send them an email using my exchange server "dtreExhcange server" this is my code

Sub SendSimpleMail(ByVal toEmailAddress As String)

        '======================================
        Dim mail As New MailMessage()
        Dim msgBody As String
        Dim smtp As New SmtpClient


        mail.From = New MailAddress("[email protected]")
        msgBody = txtEmailBody.Text
        mail.To.Add(toEmailAddress)
        mail.Subject = Subject.Text
        mail.Body = msgBody
        mail.IsBodyHtml = True

        smtp.Host = "dtreExhcange.dt.org"
        smtp.Port = 25
        smtp.EnableSsl = True
        smtp.Credentials = New System.Net.NetworkCredential("[email protected]", "123")

        Try
            smtp.Send(mail)
            mail.Attachments.Dispose()
            MsgBox("Thank You your Message has been sent.")

        Catch x As Exception
            If x.Message.ToString = "Failure sending mail." Then
                MsgBox("Please check your Internet Connection and try again", MsgBoxStyle.Information)
            Else
                MsgBox("Failed to Send", MsgBoxStyle.Exclamation)
            End If

        End Try

    End Sub

but it's not working it will not connect to the exchange server.

it will always give this error :
"Please check your Internet Connection and try again
I changed the port no but still not working .

HELP !!!

can you help me on that .