Immensely popular with corporate users due to its strong (and closed) integration with server product MicrosoftExchange via MAPI?. Outlook also talks open protocols such as IMAP and POP3. With closed-source addons such as Bynari's InsightConnector or the OpenGroupware.org plugin, you can use Outlook to do shared calendaring against a database or IMAP backend.
Criticised for a chequered security history which lead to the propogation of dozens of internet worms. Recent versions have addressed most of these issues (by default, executable attachments are blocked, images don't load in HTML emails, items are not auto previewed, etc).
REGEDIT4 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Exchange Provider] "Rpc_Binding_Order"="ncacn_ip_tcp"