The mail attachments quietly stored on your Mac

Mail is a synchronising client, which means it keeps a local copy of what is on the server so search and offline reading work. On an account with ten years of history and a habit of large attachments, that copy runs to several gigabytes, and nothing in Mail ever mentions it.

Where it lives

du -sh ~/Library/Mail
du -sh ~/Library/Containers/com.apple.mail

The first is the message store. The second is where attachments opened from a message get written, and it is the one that surprises people: opening an attachment leaves a copy behind, and nothing clears it.

Reducing it from inside Mail, which is the safe way

1. Mail, Settings, Accounts, pick the account, Account Information.
2. Set Download Attachments to Recent, or None. Recent keeps the last year or so; None fetches an attachment only when you open it.
3. In the same panel, check what the mailbox behaviours do with Trash and Junk: keeping deleted messages forever is a common and invisible cost.

Changing this does not delete anything from the server. It only changes what Mail keeps a local copy of, so a message whose attachment is no longer local downloads when opened.

The exception worth clearing yourself

The attachment scratch space inside the Mail container is not the message store, and it is safe to empty: those are copies of files that also exist in the messages themselves. Send them to the Trash rather than removing them permanently, and quit Mail first.

Why the storage bar never told you

The bar has a Mail category, but it is easy to read past because it sounds small and specific. It is also frequently wrong on accounts that use containers rather than the older store. Either way it cannot be opened, which is the general problem covered in reading the storage bar.

If Mail turns out to be small, the next places to look are the ones with the same shape: a local cache of something that also exists elsewhere. Library caches and iPhone backups are usually much larger.