Configuring preferences and options in Outlook.com
Hello SimonHutton_84,
Welcome to Microsoft Community, and thank you for reaching out!
We hope you’re having a good day despite your concern with the Outlook’s email in cache mode. Let’s work together to resolve your concern.
If you change the cache mode form 12 months to 3 months, emails that are originally saved base on the original settings shouldn’t be removed. The Outlook cache mode slider doesn’t control the size of your OST files. Instead it allowed you to select how much data is being downloaded and saved from your mailbox by age.
Please respond to this thread to let me know how the issue progresses. Similarly, if you have more questions, please inform me by replying to this post.