How do we deduce (if we can) SMTP address for an exchange account with only x500 address available? In what cases SMTP address is not available for exchange accounts (please see details below)?

I am using PR_SENDER_EMAIL_ADDRESS to retrieve sender's email address for exchange mails, but for some mails I only get the exchange X500 addresses. In such case, I can use PR_SENDER_ADDRTYPE to check the type but when I detect an X500 address instead of an SMTP one, is there a way I can enhance SMTP lookup? What might be the reasons that SMTP address is available for some External senders and not for others, as in my case? I am using MAPI CDO 1.2.1 with Exchange Server 2010.

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