The APIs for authentication are there, which is for writing a service or program is probably the most important. The other APIs are not documented but they can be reverse engineered using a browsers network console which unfortunately isn't the easiest process. However, if you are interested I have still included some documentation of the mail api as it is today.
Fields are bcc, cc, from, subject, to, messagePlainText, messageHTML
There are more but these are the common sending variables. I'd say the ones provided in my example are the bare minimum 'subject', 'to', and 'messagePlainText'.
Here is a video example