Outlook API

link - https://msdn.microsoft.com/en-us/library/office/fp161135.aspx 
link - blog.falafel.com/mail-apps-for-office-part-2-read-mode-sample 

Outlook Types

Outlook - Email reading
Outlook - Email composing
Outlook - Appointment Attendee
Outlook - Appointment Organizer

Mail

Mail add-ins require exchange 2013
You can access email message meeting request and appointment item properties and user profile information
You can also access some exchange web services features
Different activation rules (defined in manifest)
Different context triggers
Can make callbacks to the web server
Access tokens

<OfficeApp xsi:type="MailApp"> 

Properties

Methods

displayReplyAllForm
displayReplyFrom

Email message checkbox - use this if you want to create an Outlook add-in
mail items plus meeting requests, responses and cancellations

Office.content.mailbox.item - Types
message
messagecompose
appointment
appointmentcompose

Checking which mode (read or compose)

if (Office.context.mailbox.item.displayReplyForm != undefined) { 
   // read mode 
} else { 
  // compose mode 
}