in reply to [OT] MAPI.Session and NT Service Accounts
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.