We are using Odoo 13 Enterprise on premise. I created an email alias for invoices:
Aliased Model: Journal Entries
Record Thread ID: 0
Default Values: {'type': 'in_invoice', 'company_id': 1, 'journal_id': 2}
Alias Contact Security: Everyone
Owner: (empty)
Parent Model: Journal
Parent Record Thread ID: (empty)
But if an invoice with a factur-x attachment is received, the attachment is not processed. The account_facturx module is installed. If I upload the invoice manually, the factur-x attachment is processed correctly.