The problem you are having is the cdo.dll file on your BES is older than the cdo.dll file on your Exchange Server. The cdo.dll file on your BES MUST BE equal to or greater than the version on your Exchange.
To fix this, simply rename cdo.dll on your BES to cdo_old.dll and copy the cdo.dll file from your Exchange Server over to your BES.
Then on your BES go to Start --> Run --> regsvr32 "<location of cdo.dll on BES>" (must include the quotation marks) --> restart BlackBerry dispatcher services and you should be all set. (*NOTE: If you're running BES 3.6 the instructions differ slightly).