How to archive old Mac email to an external drive

Getting mail off a Mac takes a minute. Getting it off in a form somebody can still open in ten years takes three decisions and one check that almost nobody makes. The check is the important part, because the moment you delete the originals is the moment an incomplete archive stops being fixable.

The order matters more than any individual step, because one of them cannot be undone.

1. Reversible Select and export Nothing has changed. Export copies, it does not move.
2. Reversible Verify and store Count, open, check. The originals are all still where they were.
3. One way Delete the local copy On IMAP this reaches the server and every device on the account.

Three different things called archiving

Search this subject and you get three answers that all use the same word for entirely different operations. Working out which one you want takes thirty seconds and saves a great deal of confusion.

The middle one is the one most guides mean. It takes two steps. Mailbox, then New Mailbox, with Location set to On My Mac. Then select the messages, right click and choose Move To.

It catches people out. The account holds less and your Mac holds more. The Mac copy is then the only one in existence. That is tidying rather than backing up. A failed disk takes it with no warning.

The two operations pair well. Move mail to On My Mac to get under a server quota. Then export those mailboxes to a drive so a second copy exists. Doing the first without the second puts a decade of correspondence on one disk with no copy behind it.

The rest of this covers the third row, since that is the one that gets mail off the Mac and keeps it readable afterwards.

What Mac email to archive and what to leave alone

Not everything on a Mac is worth moving. Some of it should not be moved at all.

Read more: Where Apple Mail stores emails on a Mac.

The distinction is not how old the mail is or how much you care about it. It is whether a copy exists anywhere but your Mac. Everything in the left column is one disk failure from gone.

The subfolder trap in Apple Mail exports

This is the part that costs people mail. It happens quietly.

What the export command writes is a package per mailbox. Selecting a parent does not automatically bring its children. The save dialog offers an option to include subfolders. Reports of it being missing or greyed out turn up regularly. If it is not ticked or not there, you get the parent's own messages and nothing beneath them.

No warning appears either way, but the size gives it away. Miss the option and you get a package far smaller than the mail you thought you were saving. Anyone who organised years of mail into a tidy hierarchy is exactly the person this catches. The parent holds very little. The children hold everything.

The reliable route sidesteps the question. Select every mailbox in the tree rather than the top of it. Mail accepts a multiple selection and writes one package for each, so a hierarchy of fourteen mailboxes gives you fourteen packages. Flat and countable.

Choosing an email archive format that will still open

Export Mailbox writes the MBOX format and offers no alternative. Which client wrote the mail decides how much of it you can take, a split covered in how Mac email clients store your mail on disk. That is a good default. The other formats are reachable by other routes, so it helps to know what each one is for. An MBOX converter for Mac is the route to the rest of them.

Plain text is the reason MBOX ages well. Open one in a text editor thirty years from now and the messages are still legible, with or without any mail application in existence.

Getting a format other than MBOX

The last row of that table is the one to avoid for archiving. Copying the Mail folder moves you to a new Mac perfectly well. It keeps nothing safe for a decade, since it only ever opens in one application on a machine set up the right way.

Two of the others need no extra software. Dragging messages out of Mail into a Finder window writes one EML file each. That suits a handful you want to forward or attach to something. Printing a message and choosing Save as PDF produces a fixed record. That suits anything somebody has to read years later without caring about threading.

For a whole mailbox in a format other than MBOX, conversion is the route, and for loose messages that came out of Mail one by one an EML converter for Mac handles the files directly. That is a separate job from archiving and worth doing once, on the archive, rather than repeatedly on the mailbox.

Verifying the archive before you delete anything

A half finished export leaves behind something that looks exactly like a finished one. This is the step that separates an archive from a folder you hope is an archive.

1. Count before you start. Note the message count for each mailbox you are exporting. Mail shows it at the top when you select a mailbox.
2. Export, then leave it alone. Large mailboxes take longer than the progress suggests.
3. Check you have one package per mailbox. Fourteen mailboxes should give fourteen packages. Fewer means the subfolder problem caught you.
4. Look for the word partial in any filename. Mail names an unfinished export that way and never tells you. A package called partial is not an archive.
5. Read the earliest message in each package. Anything cut short loses its tail, so that is where a bad export gives itself away.
6. Try one attachment. A filename with no file behind it is the common failure. It looks fine until you click.
7. Compare counts, folder by folder. Not one total for everything. A single missing mailbox disappears inside a matching grand total.

Only now is deleting safe. Everything up to this point is reversible. The next step is not.

Where to store the archive and how to protect it

An exported archive is plain text on a disk you could leave on a train. Years of mail, readable by anyone who picks it up.

Two things fix that. Neither takes long.

• Encrypt the drive or the archive. An encrypted disk image made in Disk Utility works. So does formatting the external drive with encryption enabled.
• Make a second copy. One drive is one drive. A second copy somewhere else is what turns a file into an archive.

Where the second copy lives matters less than that it exists and is not in the same building as the first.

An archive kept on the Mac is only backed up if Time Machine can see it. Check the exclusions list before assuming. A folder excluded years ago to keep backups small is exactly where an archive tends to end up. On the Mac itself, FileVault covers the internal disk, which is why the encryption advice above is about the external one.

What to do about the copy on your mail server

If the mail came from an IMAP account, exporting has changed nothing on the server. There are now three copies. One on the Mac, one on the server, one on the drive.

Read more: What IMAP and POP actually store on your Mac and why neither is a backup.

Deleting the messages in Mail deletes them from the server too, on every device signed into that account. That is the intended behaviour and it surprises people every time.

So decide deliberately. Keeping the server copy costs you nothing on the Mac and gives you a second location for free. Removing it is the only way to reduce what the account holds. Once the archive is the only copy left, the checking you did earlier is the whole of your safety net.

Deleting the local copy from Apple Mail

With the archive verified and stored, the local mail can go. If you back up the Mac on a schedule, let one run first. That leaves a machine level copy of how things stood beforehand.

Do the deleting in Mail itself. An index sits alongside the messages. Pulling files out from under it leaves the two disagreeing about what exists. Then empty the Trash properly, which means Mailbox, then Erase Deleted Items.

Expect the space to arrive late. Both Mail and the disk lag behind a big clear out. A figure that has not budged an hour later is not proof that anything failed.

What you give up by archiving email

Two costs worth knowing before you commit, because both are easier to accept in advance than to discover afterwards.

An archive is not searchable from Mail. Once a mailbox is exported and deleted, Mail will not find anything in it, since the archive sits outside the application. Getting it back means File, then Import Mailboxes, choosing Apple Mail as the source. That restores the mail to your disk and undoes the space you gained. Fine as a one off, no way to live.

An archive also stops the moment you make it. Nothing gets added, nothing syncs, no incremental update runs. Archiving again next year means a second export and a decision about what to do with the first one.

Neither is a reason to skip it. They are reasons to archive what genuinely belongs in cold storage and leave the rest where you can reach it. That is a different decision from clearing space for its own sake.