Go to where interop.cdo.dll is located and right-click on it: Properties > Security Tab:
- Click Edit
- Click Add
- Click Advanced
- Click Find Now
Scroll down in the list and find the user that looks similiar to IUSR_<ServerName> and add it. This is your standard IIS level account. This is also assuming that you're using ASP.NET to access the interop.cdo.dll.
If it's anything else, you can follow the same steps and just use Everyone to test and see if it works. Hope it helps!
*Edit: I found this example here (adapted for c#), that might solve your problem:
CDOInteropThingy.Mailobject objCDOMail = new CDOInteropThingy.MailObject();
objCDOMail.Subject = "Sending email with CDO";
objCDOMail.From = "[email protected]";
objCDOMail.To = "[email protected]";
objCDOMail.TextBody = "This is a message.";
objCDOMail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
//Name or IP of remote SMTP server
objCDOMail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserver") = "smtp.server.com"
//Server port
objCDOMail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
objCDOMail.Configuration.Fields.Update();
objCDOMail.Send();