How Mac email clients store your mail on disk

Every mail client on the Mac keeps a local copy of what your server holds. That is why switching between them costs nothing for ordinary mail. The difference between them only matters for mail the server never had. There the gap is enormous. Some clients hand it over in a documented format. Others have no way to give it to you at all.

Features are the usual way to compare these. They are the wrong way to decide what is safe. One question settles more than a spec sheet does. If you stopped using this application tomorrow, what would you be able to take with you?

Why switching Mac email clients costs you nothing

On IMAP, Exchange, Microsoft 365, Google Workspace and Outlook.com the server holds the authoritative copy. Your client syncs a local mirror so search is fast and mail works on a train. What that mirror actually contains, plus what happens to it when the account closes, is what IMAP and POP each store on a Mac.

Delete the client, install another and sign in. The mirror rebuilds, because nothing important was ever only on the Mac.

That covers the overwhelming majority of mail on the overwhelming majority of Macs. It is why most people who switch report no trouble at all. It is also why the trouble that does happen looks inexplicable when it arrives.

The mail that does not follow you to a new client

Three categories turn up on almost every long-lived Mac. None of them is on a server.

The right column is the whole subject. It tends to be the oldest mail somebody owns, moved there deliberately to keep it safe. It is also the only mail a change of client can destroy.

What each Mac email client writes to disk

The clients fall into two generations. The split is sharper than the marketing suggests.

Read more: The hidden Library folder on a Mac and what lives in it.

The last row is honest rather than dismissive. These are server first clients by design. They cache for speed and assume your account is the archive, which is a reasonable assumption for most people. It does mean there is nothing to look inside and no documented way out. Anything you want to keep independently has to come from the server rather than the app.

The older clients write formats you can read

Apple Mail, Thunderbird and legacy Outlook were built when a client owned its data. That shows in what they leave behind.

Read more: What an MBOX file is and which Mac apps use it.

Mozilla's Thunderbird is the most portable of the three by some distance. Its local folders are mbox files by default, with maildir available instead, inside a profile directory. Copy that to another machine, another operating system or a backup drive. Read it with a text editor if it comes to that.

Apple Mail keeps its working store as individual message files rather than mbox. Its Export Mailbox command writes mbox on the way out. The destination is the same even though the storage is not.

Legacy Outlook holds records in a profile database rather than the PST people assume and offers an olm archive on export. That archive is portable in the sense that a converter reads it. What an OLM file holds explains why so little else does. It is awkward in the sense that almost nothing else does.

The newer clients assume the server is the archive

New Outlook for Mac and the newer independent clients share an approach. The server holds the mail. The app keeps whatever cache makes it fast. That cache is nobody's business but the app's.

That is defensible engineering. It removes a whole category of corruption. It makes a new machine trivial to set up. And it matches how most people actually use email now.

The cost is that there is no local-only mailbox to move mail into and no export to get anything out. Mail you want preserved independently of both the app and the provider has to come down from the server another way.

Two routes exist and both bypass the client entirely. Add the same account to a client that does export. Thunderbird or Apple Mail will download it and you export from there. Or use the provider's own export, which Google and Microsoft both offer for whole mailboxes. Neither is elegant. Both work. Both leave the client you actually like in place. For a mailbox you want kept locally whatever happens to the account, an IMAP backup tool takes the copy for you.

Can you move the store to an external drive

People ask this constantly, nearly always because an internal disk is full and an external one is not. The answer splits along the same line as everything else here.

Copying any of these folders elsewhere for safekeeping is fine. Running from that copy is the part that does not work and Microsoft says so outright for Outlook.

The reason is the same in both cases. A live mail store is a database being written to continuously. A volume that sleeps or disconnects mid write leaves it damaged, which is why the clients that allow relocation are the ones that were designed for it rather than the ones that merely tolerate a folder being moved.

What to check before you switch email clients

Five minutes of checking beforehand, in this sequence.

1. Look at the sidebar in your current client. Anything outside your account headings is local only and lives on the macOS disk rather than anywhere else.
2. If that section is empty, switch freely. There is nothing to lose and nothing to prepare.
3. If it is not, get that mail out first. Use whatever export the current app offers, while you still have the current app.
4. Check what came out before you remove anything. Counts per mailbox, not one total.
5. Only then install the new client. The server mail will look after itself.

The mistake is doing this in reverse and discovering the gap after the old client has gone. Legacy Outlook makes that especially costly, since the export it offers disappears along with the client.

Where a converter fits

Two situations need something beyond what the clients give you.

One is an archive in a format nothing current reads, an olm being the obvious case. The other is mail you want held in a neutral format for the long term, not in whatever shape one application wrote.

Both are conversion jobs rather than jobs you migrate or back up your way out of. Do them once on an archive rather than repeatedly on a live mailbox. An OLM converter for Mac handles the first case directly.

For everything else, the honest answer is that you probably do not need one. If the server holds your mail, switching client is a sign-in. That is the whole of it.