What IMAP and POP actually store on your Mac

The server holds the master copy under IMAP. Your Mac still carries a full offline cache of it, which is why Mail works with no signal. POP inverts that. The download becomes the only copy. Neither arrangement is a backup, because both die with the account.

Ask this on a forum and somebody will tell you IMAP means nothing sits on your Mac. That answer is wrong. People lose mail because of it.

The one thing the two protocols disagree about

Internet Message Access Protocol and Post Office Protocol, better known as IMAP and POP3, both move mail from a server to your Mac. Both leave a copy behind. The disagreement is about which side owns the authoritative version.

IMAP treats the server as the record. Your Mac holds a copy kept in step with it. Change something here and the change is pushed there.

POP treats your Mac as the record. The server hands a message over and then, on a schedule you set once, forgets about it.

That single difference decides everything below.

What IMAP actually puts on your Mac

The belief that IMAP stores nothing locally survives because of how Apple Mail behaves on macOS when everything works. Search returns instantly. The mailbox opens with the wifi switched off. Neither would be true if every message had to come down the wire on demand.

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

Mail writes the contents of every synced mailbox to disk. Headers, bodies and, depending on one setting, attachments.

The second version of the myth is more specific. You will read that an IMAP client pulls down only the headers and fetches the body when you click. That was a real design once, on slow links. It is not what Apple Mail does now. Pull the network and open a message from last year if you want to prove it.

Attachments are the part you control. Apple gives each account a Download Attachments setting with three values. All takes everything. Recent takes only attachments received in the past fifteen months. None takes none until you open one.

That setting is an IMAP setting. POP accounts do not have it, because a POP account has downloaded everything already by definition.

The cache is only as good as the last sync

One thing gets missed in all of this. The local copy is not a standing guarantee. It is the residue of the last successful conversation with the server.

Mail fills it lazily. A mailbox you have never opened can hold very little locally, because nothing ever gave Mail a reason to fetch it. Deep folders in a long archive are the usual example. They sit in the sidebar looking present and they are names until something goes and collects them.

Time works the same way. An account that stopped authenticating three weeks ago has a cache that stopped three weeks ago. Everything since then exists on the server alone. If the account has gone, you will not be collecting it.

So the honest description of an IMAP cache is a copy of whatever Mail happened to need, current up to the last moment it could reach the server.

The deletions you did not make

Your own deletions are the obvious case. The one that catches people is somebody else's.

Plenty of providers apply a data retention policy. Mail older than a set age is removed from the server, sometimes on a plan you chose years ago and sometimes because that is simply how the host works. Employers do the same thing with leavers and with old archives.

Because the Mac copy is a mirror, it mirrors that too. The next sync finds the messages gone upstream and removes them from the cache. Nothing warns you. Nothing asks. Mail older than the retention window quietly stops being on your Mac. The local copy was never the safeguard people assume it is.

Anything moved into an On My Mac mailbox sits outside that arrangement, because the server has no say over a mailbox it does not sync.

Why deleting on the Mac deletes on the server

This is the consequence people meet first. It surprises them every time.

On IMAP your Mac is not the owner of the mailbox. It is a participant in it. Delete a message in Mail and Mail does not quietly drop a local file. It tells the server the message is gone. The server acts on that. Your phone finds out seconds later.

Deleting is two steps rather than one. The message moves to the account's Trash, which is itself a mailbox on the server. It stays reachable there until something erases it. Erase Deleted Items is the step that takes it off the server for good.

The traffic runs both ways. Delete from webmail and it vanishes from the Mac at the next sync.

There is a sharper version involving attachments. Apple's own guidance on freeing up server space says to download attachments before stripping them, because on an IMAP account removing an attachment deletes it from the mail server and it cannot be retrieved.

What POP does differently

POP was designed for one computer holding one copy.

The Mac connects, takes the messages and then decides what to tell the server. That decision is a setting called Remove copy from server after retrieving a message. Its options run from immediately through to never.

Left at never, POP looks a little like IMAP, in that a copy survives on both sides. Set to anything else, the Mac copy becomes the only copy, on a timer you chose months ago and have long forgotten.

Nothing syncs back. Read a message on the Mac and it stays unread everywhere else. Delete it and the server never learns.

That is the appeal for people who want their mail off somebody else's computer, plus anyone whose host sells server storage by the gigabyte. A POP account does not consume a disk quota it has already emptied. It is also why a failed disk on a POP account is a different order of bad day.

What happens when the account goes

Two versions of this. One is reversible. One is not.

Turning an account off in Mail hides its messages. Apple is explicit that switching it back on shows them again without downloading them from the server, so the local copy was sitting there the whole time.

Removing the account is a different act. Apple states the outcome plainly. The account's messages are deleted and are no longer available on your Mac. Only what the server still holds remains.

Now put a closed account into that sentence. The provider shuts the mailbox. The domain lapses. An employer deactivates the address on your last day. The server copy goes. The Mac copy goes the moment the account is removed. It was never independent of the account to begin with.

Why a cache is not a backup

Apple makes this point itself, buried in the instructions for removing an account. If you are unsure whether messages you want to keep are still on the mail server, move or copy them to a mailbox stored on your Mac first.

Read more: How to archive old Mac email to an external drive.

Read that again. Apple is telling you the synced copy does not count. Keeping mail means putting it somewhere the account does not control.

A backup is a copy you can still read without the thing that made it. A cache fails every part of that sentence.

Where that leaves you

The offline cache does the job it was built for. Mail stays fast, the mailbox stays readable with no signal and none of it costs you anything to have.

It is not a second copy in any sense that outlives the address. If the mail matters beyond the life of the account it arrived at, it has to leave the account. An IMAP backup tool takes its copy from the server rather than from the cache, which is the difference between a file you own and a folder your client is borrowing.

Which client you use changes none of this. It decides only what you can extract later, which is a question about the formats each Mac email client writes to disk.