I did take this issue up w/ Microsoft Support. They are still trying
to determine if this is a bug or not. The support guy said that their
usual workaround for this issue is to use CDO, but they would not
suggest that solution to me as CDO is not supported when working w/
a .Net language as I am.

Their other workaround they tried was to use the
System.DirectoryServices namespace. Basically, parse the X500 address
to find the Exchange user name, then user DirectoryServices to look up
that Exchange user, then use the "proxyaddresses" property to get the
SMTP formatted addresses. However, the support guy could not come up
w/ a DirectorySearcher query string that would work in my environment.

Finally, he suggested that I get the X500 address, parse out the
Exchange user name, put that into a temporary ContactItem, save it,
then parse the email display name which will include SMTP address.
I'll probably do that for the short term, and then next cycle we'll
upgrade to the latest Redemption library and use the RDO method you
described above.

I'll also try getting the property via MAPI, applying your suggested
changes to the code I posted.

Jeff