If you initially setup the On Premise Engine to only scan back a short time and want to extend how far data is fetched for you'll need to follow these steps.
These steps assume you've installed the On Premise Engine at C:\sigparser folder.
Stop the scheduled task if it is running
Open the C:\sigparser\appsettings.json file
Edit the historydaystoprocess value to be a larger value (example: 365 for a year)
Save the C:\sigparser\appsettings.json file
Copy the C:\sigparser\appsettings.json file to the EmailFetcher folder (overwrite the existing file)
Delete the C:\sigparser\history folder
Start the scheduled task and re-enable it if you disabled it
If you have a lot of mailboxes it can take a long time to fetch all the emails. It will go faster for emails that have already been processed but will still take a long time. It could take days or weeks depending on how many mailboxes and how many CPU cores you have configured.