There is currently no method for inserting past data (in any format) into the the home assistant recorder database. It is set up to record data in real time - as it happens.
I guess if you were proficient enough at SQL and python you may be able to insert the data into the recorder. This would make a few folks a happy as there is a request for this about once a month. But at the moment there are no tools available to do this.
Please excuse my ignorance – I’m clearly not a programmer or HA expert.
What I was envisioning is to use the incoming daily email data essentially as a sensor (perhaps in the lay meaning of a sensor). Just as if we had a thermometer set to update every 24 hours. And then feed that data into HA for storing/manipulating/displaying.