it looks like something is causing trouble in your network (somewhere along the way from the Smtp client to the server, it might be a firewall, or antivirus software for instance).
Please first try disabling the Chunking and Pipelining extensions, which helped some of our clients in this situation (you have to use the Smtp instance for this purpose, not just the Smtp.Send static method):
Smtp smtp = new Smtp();
smtp.LogWriter = new Rebex.FileLogWriter("log.txt", Rebex.LogLevel.Debug);
// disable pipelining and chunking extensions
smtp.EnabledExtensions &= ~(Rebex.Net.SmtpExtensions.Pipelining | Rebex.Net.SmtpExtensions.Chunking);
smtp.Send("from", "to", "subject", "body");
and let us know whether you are able to send the email now with these extensions disabled. Thanks!