Yes, this is possible. Many static methods of Rebex Smtp object accept an instance of SmtpConfiguration class as one of its arguments. If you pass SmtpConfiguration.Default to it, SMTP settings from Web.config will be used:
Smtp.Send(message, SmtpConfiguration.Default);
You can also use SmtpConfiguration.Default to read the SMTP settings from Web.config and supply the values to non-static methods of Smtp object if preferred.