I have the .Net program sending an email and adding a file as an attachment

Code:
  Private Sub SendSMTP(ByVal strFrom As String, ByVal strTo As String, ByVal strSubject As String, ByVal strBody As String, ByVal strCC As String, ByVal strAttachments As String)
        Dim insMail As New MailMessage(New MailAddress(strFrom), New MailAddress(strTo))
        With insMail
            .Subject = strSubject
            .Body = strBody
            '.CC.Add(New MailAddress(strCC))
            If Not strAttachments.Equals(String.Empty) Then
                Dim strFile As String
                Dim strAttach() As String = strAttachments.Split(";"c)
                For Each strFile In strAttach
                    .Attachments.Add(New Attachment(strFile.Trim()))
                Next
                strFile = Nothing

            End If
        End With
        Dim smtp As New System.Net.Mail.SmtpClient
        smtp.Host = strSMTP
        smtp.Port = 25
        smtp.Send(insMail)

        smtp = Nothing
        insMail = Nothing
        strAttachments = Nothing

    End Sub
When I tried to erase the file at the end it says the the file is still in use. I tried declaring the variables to nothing and still doesn't help. If I dont send the file in the email I can erase it with out a problem. Any suggestions?