Thanks for all the suggestions - I've now managed to make it work.

By copying CDO.DLL into C:\WINNT\SYSTEM32 and registering that (from Exchange Admin) the class becomes available (even though it claimed to be there before!).

I needed to set Win32::OLE->Initialize(Win32::OLE::COINIT_OLEINITIALIZE) in a BEGIN block to match up the threading model though.

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Mark