CDOsys.dll to send emails stopped working on Windows Server

Question

Using CDOsys.dll to send emails through Unisys application called Mapper. The emails have stopped working on a server. Below is the error message: Tried many different changes within IIS and IIS(6).
Reregistered cdosys.dll

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:: Register 64-bit CDO

%windir%\System32\regsvr32.exe %windir%\System32\cdosys.dll

:: Register 32-bit CDO

%windir%\SysWOW64\regsvr32.exe %windir%\SysWOW64\cdosys.dll``

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Validated it worked using: :C:\Windows\System32\cscript.exe //nologo cdo_test.vbs

Prior to this working the server was upgraded from Windows 2016 to Windows 2019.

Answer 1

Hi Tom Morris,

The error you’re seeing usually points to a COM registration or compatibility issue, especially after moving from Windows Server 2016 to 2019.

You’ve already re‑registered both the 32‑bit and 64‑bit versions of cdosys.dll, which is the right first step. The next thing to check is whether your application is running in 32‑bit or 64‑bit mode, because mismatches there can cause the COM object not to load correctly. If Mapper is running as a 32‑bit process, make sure it’s explicitly calling the SysWOW64 registration.

Another important step is to verify that the IIS SMTP service or any mail relay you’re using is properly configured on Server 2019. CDO relies on the local SMTP service or a defined smart host, and after an upgrade those settings sometimes reset. You can test this by creating a simple VBScript using CDO.Message and pointing it to a known SMTP server to confirm whether the library itself is working.

If the script works but Mapper fails, then the issue is likely with how the application is binding to the COM object. In that case, check the application’s configuration files or registry entries to ensure they’re pointing to the correct class ID. If the script also fails, then you may need to repair or reinstall the IIS SMTP feature, or apply the latest cumulative updates for Server 2019, which include fixes for COM and mail components.

I hope the response provided some helpful insight. If it addressed your issue, please consider marking it as Accept Answer so others facing the same problem can easily find the solution. If you need any further assistance, feel free to leave a comment.

Jason.