using(MailMessage message = new MailMessage("john.smith@gmail.com"){
    MemoryStream stream;
    //here I pass byte array to the stream and create an attachemnt
    message.Attachments.Add(new Attachment(stream, "name.xyz"));

    using(SmtpClient client = new SmtpClient("server.com", port))
    {
        // send message
    }
}

try
{
    client.Send(messgae);
}
finally
{
    if(stream != null)
        stream.Dispose();
}