Outlook and MAPI Developer Resources:
Outlook for Developers forum - forum is for Developer discussions and questions involving Microsoft Outlook.
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 since 1999, but you can still 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 - (updated October 1, 2011) 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)