Mac storage basics · Mail, Messages and chat apps
How do I remove attachments from emails in Apple Mail but keep the messages?
Short answer: Select the messages and choose Message → Remove Attachments. On server-synced accounts the attachments may be removed from the server too, so save what you need first.
In Mail, select one or more messages and choose Message → Remove Attachments. The messages stay; the attached files are removed.
Be careful with accounts that sync with a server: the change can apply on the server too, so the attachment may disappear everywhere. Save anything you need first (drag it to a folder in Finder). For a lighter touch, Mail's account settings can stop it downloading attachments automatically — see stopping Mail downloading attachments.