If you don't want to use the local storage on your server where the On Premise engine is deployed you can instead use Azure Storage. Be careful the storage account is not public.
The Azure Storage option has pros and cons. The pro is the storage can grow forever but you are unlikely to need that much storage because the storage is only used when the SigParser API is unreachable which isn't often. The con is that it is harder to setup and managing connection strings becomes an additional burden.
Create an Azure Storage Account (optional)
In Azure Portal, search for Storage accounts
Choose the appropriate subscription & resource group
Specify the storage account name, something like sigparserstorage
Choose StorageV2 (general purpose v2)
Choose Locally-redundant storage (LRS)
Choose Next to proceed to Networking
Choose Public endpoint(all network)
The storage account's contents should not be made public. This settings just controls the endpoint for this storage account.
You can make a private endpoint but it requires a private DNS and a network interface (NIC). See this page for details.
Click Review + create
Give it a minute to deploy
Click Go to resource
Confirm public access to the storage account is disabled.
Go to Settings -> Configuration and verify Allow Blob public access is Disabled
On the left menu bar, click File shares
Click + File share
Choose some name, like sigparsershare
Save this name into the EmailFetcher appsettings.json config file as azurestoragesharename
Choose the Hot tier
On the left menu bar, click Access keys
Click Show keys at the top of the page.
Copy key1’s Connection string
Go back to the Azure Vault page
Under Settings, click Secrets
Enter AzureStorageConnectionString as the name
Paste the connection string from the previous step as the value