Why Apple Mail takes up so much disk space

Apple Mail keeps two separate stores and most people only ever measure one. The message store holds your mail. A second folder inside the application container holds a copy of every attachment you have ever opened. It survives deleting the messages those attachments came from.

That is why the familiar sequence fails. Somebody deletes thousands of messages, empties the Trash, checks the size and finds it unchanged. Nothing is broken. They cleared the smaller of the two folders.

Why the Apple Mail folder grows in the first place

Three behaviours run at once and none of them is a fault.

• Mail caches everything for offline use. By default it downloads and keeps every message on every account, so the whole mailbox works with no connection.
• Attachments come down with the messages. The cache is not headers and subject lines. It is the full content, images and video included.
• Every attachment you open is copied a second time. Previewing a PDF writes it into a separate folder, which nothing later removes.

On a Mac with a large disk none of this matters. On a 256GB MacBook Air with eight years of mail across three accounts it matters a great deal. The growth is invisible because no single action caused it.

The two Apple Mail folders you have to measure

This is where most diagnosis goes wrong. Mail occupies two locations and a figure from either one alone is an underestimate.

~/Library/Mail

~/Library/Containers/com.apple.mail

Check both. Paste each path into Finder's Go to Folder box, select the folder itself rather than opening it, then press Command and I for Get Info. Note each size and add them together. Where Apple Mail stores emails on a Mac walks through what is inside each one.

Add the two together before drawing any conclusion. A mailbox reported as twelve gigabytes in one place can be considerably larger once the container is counted. The container is the half people never look at.

Mail Downloads, the folder nothing cleans

Inside the container sits a directory that explains most complaints about this subject.

~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads

Every attachment you open or preview is written here as a separate file, each in its own subfolder with an alphanumeric name. Not moved, copied. The original stays with the message and this one stays behind after the message is gone. Quick Look counts as opening it.

Check whether it exists on your Mac before planning around it. The folder has been there for well over a decade and is still reported on current systems, but it is not guaranteed. At least one report on Sonoma has it absent entirely. If the path returns nothing, that half of the problem does not apply to you.

Years of invoices, screenshots, newsletter images and one large video somebody sent in 2019 accumulate with no upper bound and no housekeeping. Deleting mail does not touch it. Emptying the Trash does not touch it. Nothing in Mail's interface refers to it at all.

You can prove it in two minutes. Send yourself a message with an attachment, open the attachment, then delete the message and empty the Trash. The copy is still in the folder. It is not a cache either, whatever it looks like. Which Mac caches are safe to clear draws that line properly.

Why deleting emails does not shrink Apple Mail storage

Four things stand between deleting a message and seeing the number fall.

Deleted and junk messages also keep counting until they are erased rather than merely deleted. Two commands on the Mailbox menu do that. Erase Deleted Items handles the Trash. Erase Junk Mail handles the other one.

None of this is a defect. It is the consequence of a design that assumes disks are large and connections are unreliable, which was a reasonable assumption when the behaviour was written.

How to find which account is using the space

Once you know the total, the useful question is which account is responsible. Almost always it is one of them rather than all of them.

1. Open the message store and sort by size. In Finder, switch to list view, then add the Size column if it is not showing.
2. Wait for the sizes to finish calculating. Folders this large report nothing useful for the first minute.
3. Open the largest account folder. Its name tells you nothing, so work out whose it is from the mailboxes you find inside.
4. Check On My Mac separately. Local mailboxes hold the only copy of what is in them, so they change what your options are.
5. Now measure Mail Downloads. If that folder is the larger figure, the accounts were never the problem.

On the forums the answer is frequently On My Mac. Somebody who moved years of archived mail out of an inbox to keep it safe ends up with the only copy sitting on the disk they are trying to clear. Getting that mail somewhere durable is the job worth doing before anything is deleted.

There is a quicker route from inside Mail if you only want the worst offenders rather than a per account total. Choose View, then Sort By, then Size. The heaviest messages rise to the top of whichever mailbox you are in.

Why space you cleared comes back

Clear cached messages on the disk and they can reappear after the next sync, because the server still holds them and Mail's job is to keep the two in step.

This catches people who delete inside the message store in Finder rather than inside Mail. The disk gets smaller for an afternoon, then Mail notices the gap and downloads it all again. Cached attachments in the container do not come back that way, since nothing is syncing them, which is part of why that folder is the more useful target. Space that refuses to return at all is a different problem, covered in purgeable space and the snapshots holding it.

The one command that is not reversible

Mail has a Remove Attachments command, on the Message menu. It strips attachments from a message and leaves the message behind, which sounds exactly like what somebody clearing space wants.

On an IMAP account it removes them from the server too. Apple states this plainly in its own guidance. Deleting cached files in Finder leaves the server copy alone, because those are duplicates. Remove Attachments is not that. It is a change to the mailbox rather than to a cache. It syncs. The attachments cannot be fetched back. Download anything you want to keep before using it.

That distinction runs through this whole subject. It is the one thing worth carrying away from it.

Done in Finder

Touches copies

• Clearing the Mail Downloads folder
• Emptying the container caches
• Server keeps its copy either way
• Reversible, because it re-downloads

Done inside Mail

Touches the mailbox

• Deleting messages
• Erase Deleted Items and Erase Junk Mail
• Message, then Remove Attachments
• Syncs to the server on IMAP, so it is permanent

The Apple Mail setting that stops it growing

Mail can be told not to download attachments automatically, which stops the message store from growing at the rate it has been.

The option is in Mail settings, under Accounts, on the Account Information tab for each account. Download Attachments offers All, Recent or None. Recent keeps the last few weeks and fetches anything older on demand. None fetches everything on demand.

Two things worth knowing before changing it. The setting is per account, so an account you never touch is worth setting to None on its own. And it changes nothing about what is already on the disk, which has to be dealt with separately.

The POP account exception

Everything above assumes IMAP, where your Mac holds a working copy of something the server keeps. POP inverts that.

Read more: What IMAP and POP actually store on your Mac.

On POP, local files can be the only copy. Messages are pulled down and removed from the server afterwards, so deleting cached attachments is not clearing a duplicate. It is deleting the original. Check Mail settings, Accounts, Advanced. Look at what the option to remove copies from the server is set to before you delete anything.

How much space Apple Mail should be using

Ten to forty gigabytes is ordinary for an account of several years with attachments. Well past a hundred happens on a mailbox nobody has ever pruned. It is still not evidence of a fault.

The figure only matters against your free space. On a 2TB disk a 60GB mail store is a curiosity. On a 256GB MacBook Air it is a quarter of everything you own and worth doing something about.

What that something is depends on whether the mail matters. Cached attachment copies on an IMAP account are duplicates. Clearing those in Finder costs you nothing. On My Mac mailboxes are the opposite case. They want moving somewhere durable before anything is deleted, which a Mac MBOX converter does by reading the store directly. That is a different job from cleaning up. To see how Mail compares with everything else on the disk, start at how the rest of the disk is being used.