How to Fix Invalid SendUsing Configuration using CDO

When you use CDOsys to send email from a classic ASP, you might see an error message below and if you look at the line error, it’s always on the message.Send:

The page cannot be displayed
There is a problem with the page you are trying to reach and it cannot be displayed.
Please try the following:
Contact the Web site administrator to let them know that this error has occurred for this URL address.
HTTP 500.100 - Internal server error: ASP error.
Internet Information Services
Technical Information (for support personnel)
Error Type:
CDO.Message.1 '80040220'
The "SendUsing" configuration value is invalid.
/path/filename.asp, line 100
Browser Type:
Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.71 Safari/537.36
Page:
POST 279 bytes to /path/filename.asp
POST Data:
category=...
Time:
Monday, October 26, 2015, 10:12:55 PM
More information:
Click on Microsoft Support for a links to articles about this error.
Go to Microsoft Product Support Services and perform a title search for the words HTTP and 500.
Open IIS Help, which is accessible in IIS Manager (inetmgr), and search for topics titled Web Site Administration, and About Custom Error Messages.
In the IIS Software Development Kit (SDK) or at the MSDN Online Library, search for topics titled Debugging ASP Scripts, Debugging Components, and Debugging ISAPI Extensions and Filters.

Several posts would suggest that you put the SMTP server in the code. But when you already have an SMTP server setup on IIS 6.0, here’s a couple things you might want to check as well:

1. Check if the Application Pool is at least running under version 2.0.

2. Give user MACHINE\IIS_IUSRS modify permission on C:\inetpub\mailroot directory (or if you moved the SMTP folders somewhere else, give that permission to the new directory instead).

   

Also check the links under Further Reading below for the common solutions pertaining to this error.

Further Reading

The “SendUsing” configuration value is invalid. (IIS.NET Forums)
The “SendUsing” configuration value is invalid. (ASP.NET Forums)