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.
About: Office 365 REST APIs for Exchange
About: Exchange Web Services (EWS)
About: Outlook Object Model (OOM)
About: Extended MAPI
About: Exchange Server ActiveSync (EAS)
HOWTO: Read unmounted Exchange EDB files.
Directly accessing Exchange database files is NOT supported. There are specific supported APIs for accessing data.
"Enhancing Exchange" via unsupported methods.
About: Messaging APIs