Is it possible to create an add-on that can send off the contents of an email into a webservice? I see lots of documentation on add-ons interacting with external webservices and have been able to follow a simple demo about it, but there isn't much I can find about interacting with the emails itself.
I have an add-on in Outlook that grabs the email and sends it off to a webservice, but I want to replicate this in Gmail in some way.
Next, I would also like to know if it's possible to use a different web service based on the user. For example, if I were to deploy this same add-on for a coworker but they needed to send the email to the different service. Is there a configuration file somewhere that controls this?
Lastly, I was also interested in how Gmail add-ons store personalized information, as I feel like it would help clear things up. E.g. how the tasks add-on grabs the right ones for the currently logged in user and where they are stored.
I would be open to any other solutions to this issue.
Communicating with web service
You can use the UrlFetchApp class to connect to any target web service if it can be accessed via HTTP request. If you need to access different web services for different users, you can either do so via storing "user:url" property (see below) or by conditionally changing the URL according to effective user.
Determining the user
You can determine under whose authority the Add-on is running by using a Session class method getEffectiveUser() and getEmail() on the resulting user.
Storing personalized info
As for storing and fetching personalized information, there is a PropertiesService class you can use. Since you want to store info related to end users, you'll need to get a Properties class instance with access to user-specific properties via getUserProperties() method.
Useful links