User-defined type not defined

I am getting the message "User-defined type not defined" in the following line:
Dim objSession As MAPI.Session

I am running it in Windows 7 and in Visual Basic there is no reference missing.

Thank you in advance for any solution.
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Rikin ShahMicrosoft Dynamics CRM ConsultantCommented:
try with-

Dim objSession
Set objSession = CreateObject("MAPI.Session")

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Om PrakashCommented:
You need "Microsoft CDO 1.21 Library" as a reference for MAPI
DistillingExpertsAuthor Commented:
Thank you. But why this solution works?
Dim objSession
Set objSession = CreateObject("MAPI.Session")
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Visual Basic Classic

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