Maybe I'm misunderstanding you, but note that this plugin does not deal with inboxes, sparkpost is used to send email not to receive it.
Using this plugin you are able to send emails in exactly the same way described in the web2py book in the sending section:
The objective was to make it as easy as possible for web2py users to use the sparkpost REST API without having to change any kind of code in their project.
I'm open to adding missing features if people need them as I will also be using it.