Apple Mail can quietly consume 20-100 GB of a Mac's internal drive because, unlike a webmail client in a browser, it downloads real copies of your inbox, including every attachment, to disk. If your Mail app is huge, the cause is almost always years of downloaded attachments and signature images cached locally, not your actual messages, which are tiny.

TL;DR

  • Apple Mail downloads full copies of messages and attachments to ~/Library/Mail, where they hide from normal Finder views.
  • The bloat is overwhelmingly downloaded attachments and logo-heavy signatures.
  • The safe fix: change Download Attachments to None or Recent, then let macOS reclaim space.
  • The fast fix: remove and re-add the account; mail lives on the server, so nothing is lost.
  • Removing a Mac account only deletes the local cache, never the server copy.

Why does Apple Mail grow so large?

By default, Mail downloads every attachment it encounters and stores it on disk so messages open instantly offline. After a few years on a busy account, with heavy PDFs, slide decks, and signature images repeated in thousands of threads, that local cache becomes enormous. All of it lives deep inside ~/Library/Mail, a hidden folder that does not appear in ordinary Finder browsing, which is why the space seems to vanish into the Mac's System Data without an obvious cause. The messages themselves are mostly text and take almost no room; the attachments are the weight.

The safe fix: change Download Attachments

This stops new attachments from filling the disk while keeping every message available:

  1. Open Mail, then click Mail › Settings in the menu bar.
  2. Go to the Accounts tab.
  3. Select your account in the left sidebar.
  4. Under Account Information, change Download Attachments from All to None or Recent.

With None, Mail fetches an attachment only when you open it; with Recent, it keeps the latest ones. Existing cached attachments shrink over time as macOS reclaims them, especially with Optimize Mac Storage enabled in your Apple ID settings.

The fast fix: re-add the account

Because your mail actually lives on the server (IMAP, iCloud, Exchange, Gmail), removing an account from the Mac deletes only the local cache, not a single real message:

  1. Open Mail › Settings › Accounts.
  2. Select the bloated account.
  3. Click the minus (−) button to remove it from this Mac.
  4. Wait for the local data in ~/Library/Mail to clear.
  5. Click the plus (+) button and add the account back.
  6. Before the full sync starts, set Download Attachments to None.

The account re-syncs from the server with a clean, lean local cache.

Safe fix vs fast fix: which should you use?

Approach Space freed Speed Best for
Change Download Attachments Gradual, as macOS reclaims Slow but hands-off A quick setting change you can forget about
Remove and re-add account Immediate, full local cache Fast but resyncs Reclaiming a large chunk of disk right now

Is it safe to delete Mac Mail's storage?

Yes, with the methods above. Both changing the download setting and removing the account affect only the local cache on your Mac; the authoritative copy of every email stays on the mail server and re-downloads on demand. What you should not do is manually drag folders out of ~/Library/Mail in Finder, as that can corrupt Mail's local database and force a slow, error-prone rebuild. Stick to Mail's own Settings, and enable Optimize Mac Storage so the system manages reclamation for you. If you rely on offline access to old attachments, choose Recent rather than None.

FAQ

Why does the Mac Mail app use so much storage?

Apple Mail downloads full copies of your messages and, by default, every attachment to ~/Library/Mail on disk. Years of PDFs, slide decks, and signature images make that local cache balloon to tens of gigabytes, even though the messages themselves are tiny.

Will I lose emails if I remove the account from my Mac?

No. Your mail lives on the server, so removing the account from your Mac deletes only the local cache. When you add the account back, every message re-syncs from the server intact.

How do I stop Mail from downloading attachments?

Open Mail › Settings › Accounts, select your account, and under Account Information change Download Attachments to None or Recent. Mail will then fetch attachments only when you open them.

Where does Apple Mail store its data?

Apple Mail stores messages and attachments in ~/Library/Mail, a hidden folder not shown in normal Finder views. This is why the space often appears under the Mac's System Data category with no obvious source.

Why doesn't my Mac free the space immediately after changing settings?

macOS reclaims cached attachment space gradually rather than instantly. Enabling Optimize Mac Storage in your Apple ID settings speeds this up; for an immediate result, remove and re-add the account.

If Mail is lean but the drive is still full, the next suspect is usually the catch-all System Data bucket. Continue with the guide on the mysterious System Data on Mac and how to safely clean the disk. For the broader cleanup mindset and what is safe to remove, see the truth about cleaner apps and whether they are safe to use and what to back up before cleaning your device. For a starting point on reclaiming storage system-wide, browse the clean up phone storage solution hub.

For your phone, Cleanor for iPhone finds large videos, duplicate photos, and heavy caches in one on-device pass — nothing uploaded.