Microsoft product (line). [Explain. Give references.] [There are several distinct flavors--"IMO" just does some kinds of e-mail ("Internet Mail Only"), "Corporate/Workgroup" gives ... with CDO ...]
http://insideoe.tomsterdam.com/ has much information about Outlook and WAB (Windows Address Book).
Outlook Express does not expose a COM interface, but Outlook 2000 does (true?). Even in the case of the former, [L1 ] apparently offers a way to hack COM (also see vbaoutl.hlp on msdn.microsoft.com).