Since this year (2019) Google will have more strict policy regarding the Gmail scope. If in the past for sending notifications we were used to request the Send Email scope to user, now it is less comprehensible and on Google side you will have to provided lot of more details and your application can be rejected. So it become time for thinking to send email as the application and not as the user. Say Hello to RelayApp service
If you are after performance and self cleansing then the CacheService is the best solution for caching, and if you are after permanence and small amounts of data, the properties service is a good solution. You can also get round many of the limitations of each service using my properties and cache plugins, covered in Squeezing more into (and getting more out of) Cache services
In a recent episode of Totally Unscripted I demonstrated how you can use the Dialogflow service as a way to allow you to add conversational interfaces to your Google Apps Script projects. In this post I share some boilerplate and instructions for implementing Dialogflow into G Suite editor add-on projects