I've assembled lists of sites where you can look for information and help on APIs dealing with developing applications against Exchange and Outlook.
Messaging Development training:
Exchange Server development
Welcome to the Outlook 2013 developer reference
This is Microsoft's video based training site. It covers a broad range of developer topics including messaging APIs.
Developer Support Team blogs:
Developer Support Messaging Team
EMEA Developer Support for Messaging Team Blog
Exchange product team blogs:
Exchange Dev Blog
Exchange Team Blog
Office Developer Documentation
Office client development
Glen's Exchange Dev Blog
This is one of my favorite sites for Powershell scripts.
Glen Scale's Exchagne Column
Exchange development forums
Support - All products