Mail is taking up space on a Mac: attachments, downloads and the index

Where Mail keeps things

• ~/Library/Mail — the mailboxes, inside a folder named V followed by a number. Every message is a file, with its attachments inside it. For an iCloud, Gmail or other IMAP account this is a copy of what the server holds.
• ~/Library/Mail/V…/MailData — the index (Envelope Index), which is what makes search and sorting fast. It is built from the mailboxes and can be rebuilt.
• ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads — a second copy of each attachment you have opened or Quick Looked. The original stays in the message.

To see how they compare:

du -sh ~/Library/Mail ~/Library/Containers/com.apple.mail/Data/Library/Mail\ Downloads 2>/dev/null

Terminal may need Full Disk Access (System Settings → Privacy & Security) to read either. System Settings → General → Storage shows Mail's total under its own heading.

Clear Mail Downloads

This folder is the safe win. Everything in it is a copy, so clearing it loses nothing: the attachment is still in the message, and opening it again makes a new copy.

1. Quit Mail.
2. In Finder, press Shift-Command-G and go to ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads.
3. Select everything inside and move it to the Trash. Delete the contents, not the folder.
4. Empty the Trash.

Then stop it refilling: Mail → Settings → General → Remove unedited downloads, set to When Mail Quits or After Message is Deleted rather than Never. An attachment you edited and saved is kept either way, which is the point of the wordunedited.

Download fewer attachments

The mailboxes themselves are mostly attachments: a few years of mail is small, the PDFs and photos sent with it are not. Mail → Settings → Accounts → select the account → Account Information → Download Attachments. It does not cover every attachment: Mail always downloads pictures, PDFs, video and audio. The setting decides what happens to the other types, such as spreadsheets and zip files:

• All — always downloaded.
• Recent — downloaded only if received within the past 15 months.
• None — not downloaded until you open them.

Recent or None keeps the mail and gives back the space those other attachments take, at the cost of a short wait the first time you open an old one. For a mailbox that is mostly photos and PDFs, the space comes back only by deleting the messages, or by saving the attachments and choosing Message → Remove Attachments. A POP account keeps everything it has fetched.

Erase deleted and junk messages

A deleted message sits in the account's Trash mailbox, at full size, until it is erased.Mailbox → Erase Deleted Items clears it now, for one account or all of them, and Mailbox → Erase Junk Mail does the same for Junk. To make it automatic: Mail → Settings → Accounts → the account → Mailbox Behaviours → Erase deleted messages, set to one month, one week, one day, or when quitting Mail. For an IMAP account the erase reaches the server too, so the message is gone from your other devices as well.

Rebuild the index

The index grows as mail does and is tidied as messages go, so it is rarely the problem. When search gives wrong answers or a mailbox shows the wrong count, Mailbox → Rebuildrebuilds one mailbox; for an IMAP account that means downloading its messages again, which takes time rather than space. To rebuild the whole index, quit Mail, move the Envelope Index files out of ~/Library/Mail/V…/MailData to the Desktop, and open Mail, which imports every mailbox again and builds a fresh index. Delete the old files once it has finished and search works.

If the account is the problem

For an IMAP account the Mac's copy is a copy. Removing the account from this Mac (System Settings → Internet Accounts → the account → untick Mail) deletes the local mailboxes and nothing on the server; add it back later and Mail downloads what the Download Attachments setting allows. That is the way to reclaim a mailbox you no longer read on this Mac. Deleting messages from a POP account, by contrast, is the only copy going.

Why it grows back

Because Mail keeps downloading what the account settings tell it to, and because every attachment you open is copied to Mail Downloads again. The settings above are what change the rate: the folder you clear by hand is empty for as long as it takes to open the next PDF.

Where the app comes in

RefreshYourMac measures Mail Downloads as its own category, rated Safe for auto removal, because every file in it is a copy whose original stays in your mailbox. Clean All Safe takes it with the other regenerable categories. A scheduled clean leaves it alone while Mail is open; a clean you start yourself is never blocked. It is a download-edition category; the Mac App Store edition does not have it.

The mailboxes and the index in ~/Library/Mail are not a category: they are your mail, and the ways to make them smaller are Mail's own settings above. For the other places worth checking, see how to free up space on a Mac, or Messages, whose attachments are the same story in a different folder.