Complex error sending HTML email by system.web.mail vb.net

Question

I am getting following error while trying to send HTML email trough my vb.net code(I am using system.web.mail). I have mailenable enterprise edition installed on my server.

I have vb.net window application which can successfully send HTML emails for most of the emails. But for some emails only i am getting following exception.

Exception : Unknown Error
Stack Trace : at System.Web.Mail.SmtpMail.LateBoundAccessHelper.SetProp(Object obj, String propName, Object propValue)
   at System.Web.Mail.SmtpMail.CdoSysHelper.Send(MailMessage message)
   at System.Web.Mail.SmtpMail.Send(MailMessage message)
   at Scheduler.MainScheduler.main() in E:\Projects\KG\scheduler\MainScheduler.vb:line 620

Inner Exception : Exception has been thrown by the target of an invocation.
Inner Exception stack trace: at System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters)
   at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)
   at System.Web.Mail.SmtpMail.LateBoundAccessHelper.SetProp(Type type, Object obj, String propName, Object propValue)
   at System.Web.Mail.SmtpMail.LateBoundAccessHelper.SetProp(Object obj, String propName, Object propValue)

Can anybody help me to find out problem? Is it related to outbound size of email? Whats the default outboud email size in mailenable enterprise edition? I have the "Limit outbound message size" checkbox is unchecked.