About: Mailbox Accessing APIs

These APIs are used to access folders and items inside mailbox and public folder stores.  These are the APIs used to create mail client applications.  They have varying levels of business logic available - not all APIs have objects for working with specialized item types like contacts, appointments, tasks, etc. These APIs do not do administrative tasks and do not send email directly by SMTP.

APIs

About: Office 365 REST APIs for Exchange
http://blogs.msdn.com/b/webdav_101/archive/2015/05/18/about-365-rest-apis.aspx

About: Exchange Web Services (EWS)
http://blogs.msdn.com/b/webdav_101/archive/2015/05/03/getting-started-with-ews.aspx

About: Outlook Object Model (OOM)
http://blogs.msdn.com/b/webdav_101/archive/2015/05/18/about-outlook-object-model-oom.aspx

About: Extended MAPI
http://blogs.msdn.com/b/webdav_101/archive/2015/05/18/about-extended-mapi.aspx

About: Exchange Server ActiveSync (EAS)
http://blogs.msdn.com/b/webdav_101/archive/2015/06/01/about-exchange-server-activesync-eas.aspx

Old APIs:

About: CDO 1.21
http://blogs.msdn.com/b/webdav_101/archive/2015/05/18/about-cdo-1-21.aspx

About: WebDAV
http://blogs.msdn.com/b/webdav_101/archive/2015/05/18/about-webdav.aspx

Avoid these:

HOWTO: Read unmounted Exchange EDB files.
http://blogs.msdn.com/b/webdav_101/archive/2008/09/03/howto-read-unmounted-exchange-edb-files.aspx
Directly accessing Exchange database files is NOT supported.  There are specific supported APIs for accessing data.

"Enhancing Exchange" via unsupported methods.
http://blogs.msdn.com/b/webdav_101/archive/2013/06/13/quot-enhancing-exchange-quot-via-unsupported-methods.aspx

Also see:

About: Messaging APIs
http://blogs.msdn.com/b/webdav_101/archive/2015/08/07/about-messaging-apis.aspx