Sending, receiving, and organizing email in Outlook.com
It sounds like you're experiencing a complex issue with your Outlook.com mailbox where the content of new emails is being incorrectly displayed. This could indeed be related to some form of content index corruption, especially since you mentioned that the message headers remain correct while the body displays old spam content.
Unfortunately, the context provided does not contain specific guidance on reindexing mailboxes or resetting mailbox databases for Outlook.com. Typically, such actions would need to be handled by Microsoft support or a technician with access to the backend systems.
Since you've already taken several steps to secure your account and eliminate potential causes, I recommend reaching out to Microsoft support directly for assistance. They can provide the necessary tools and support to investigate and potentially resolve the issue with your mailbox.
References: