Quit Mail
Allow synchronization to finish, then quit Mail before scanning its supported attachment cache.
Grant access
Approve the requested Home or Mail-data folder authorization. Start a new scan after access changes.
Check local availability
An attachment stored only on the mail server may not have a local cache to find. Open the message and download it when you intentionally want it available offline.
Account and version differences
Mail storage layout can change with macOS, account type, and migration. An account removed from Mail may leave data in another state.
Empty result
An empty result can be correct if Mail or macOS already removed the cache. It does not mean original messages or server attachments are missing.
Do not alter Mail’s internal folders manually to force detection.