Outlook and MAPI Developer Resources:

CDOLive - an excellent source for undocumented MAPI properties and CDO tricks by Siegfried Weber.

MailFAQ - a list of tips and tricks maintained by Daniel Mitchell.

Slipstick - lots of developer tips and tricks; huge list of third-party Outlook solutions.

MSDN @ Microsoft - the very first place to look if you have any Outlook or Extended MAPI questions.

MAPI Developers Forum - Extended MAPI developers mailing list. Great place to ask your Extended MAPI questions (~500 subscribers).

Outlook-Dev mailing list - The Outlook Development mailing list is aimed at developers working with Microsoft Outlook both with and without a Microsoft Exchange Server environment. (~700 subscribers).

Inside MAPI - The only book on Extended MAPI ever published. It has been out of print for about three years now, but you can buy its PDF version on a CD. Don't even think about doing any serious Extended MAPI development without this book!

Extended MAPI headers for Borland Delphi - original Extended MAPI are supplied in C/C++ only. Alexander Staubo translated most of these header files to Delphi, I added missing pieces (mostly in Exchange Server support area). Not all the headers are translated, but they should cover 99.9% of the typical Extended MAPI projects.

TNEF stream reader Delphi class. A class to read and extract all properties from a TNEF stream (the infamous winmail.dat)