CDO 1.21 (that is what "MAPI.Session
" is) is no longer being installed by Outlook. You can download it from https://www.microsoft.com/en-us/download/details.aspx?id=3671, but keep in mind that it is no longer being developed or supported by Microsoft.
What do you do with CDO 1.21? You really need to change your code to use only the Outlook Object Model. You can also use Redemption (I am its author) - it's RDO family of objects is (just like CDO 1.21) an in-proc MAPI wrapper and its RDOSession object is similar to MAPI.Session
in CDO 1.21 (but has quite a few extra goodies).