Switching email clients on a Mac, what carries over and what does not

On an IMAP account the messages and folders move themselves, because they live on the server rather than on the Mac. Everything else stays behind. Local folders, rules, signatures, smart mailboxes and account settings all belong to the client you are leaving. No protocol carries them across.

An email client migration on macOS takes ten minutes. Add the account to the new client, wait for it to sync, done. What goes wrong happens weeks later, when somebody looks for a folder that was never on the server in the first place.

What IMAP carries on its own

The server holds the record, so anything the server knows about arrives in the new client without you doing anything.

IMAP defines those flags itself. Seen, Answered, Flagged, Draft and Deleted are defined in IMAP and stored on the server, which is why a message you read on the old client shows as read on the new one.

Coloured flags are a different thing. IMAP also allows a client defined keyword, which is what Apple Mail's colour labels use. Keywords are not supported by every server. Where the server drops them, the colours simply are not there afterwards. Nothing warns you.

A switch is two jobs, not one

Separating them makes the whole thing tractable.

The first job is the account. Add it to the new client and let it sync. The server hands over everything it holds. This part is automatic and it is the part every migration guide describes.

The second job is the residue. Every client accumulates things that never went to a server. Either you put them there deliberately. Or the client had nowhere else to keep them. You move that material as files, by hand, before the old client goes.

People treat a switch as the first job and discover the second one afterwards. Doing them in the wrong order is what makes the loss permanent, because the old client goes the moment the new one looks like it works.

What never travels

This is the list that matters, because none of it produces an error message. It is simply absent.

Local folders. On My Mac mailboxes in Apple Mail, On My Computer folders in Microsoft Outlook, Local Folders in Thunderbird. Different names for the same thing, mail somebody deliberately moved off the server. The new client has no way to know it existed. This is the single most common loss on any switch.

Rules. Every client stores them in its own format and applies them locally. There is no export that another client reads. They have to be rebuilt by hand. A mailbox with twenty years of filing rules is a long afternoon.

Signatures. Client side, stored in the client's own settings, gone the moment you stop using it. Copy the text out before you uninstall anything.

Smart mailboxes and saved searches. These are queries rather than folders. They hold no mail, they describe mail. The description belongs to the client that wrote it.

Account settings. Server names, ports, security settings and any custom port your host uses. Note them down first, because a client that has forgotten them cannot be asked.

Anything on a POP account. A POP mailbox was downloaded rather than synced, so the server has nothing to hand the new client. What matters here is which of the two protocols your account actually uses, since the answer changes the entire job.

Contacts and calendars are not email

People expect these to arrive with the mail and they do not, because they never travelled on the mail protocol at all.

Contacts sync over CardDAV. Calendars sync over CalDAV. Exchange and Microsoft 365 use their own protocol for all three at once. Add an account to a new client and whether contacts arrive depends on that client speaking the right protocol to that provider. IMAP working correctly has nothing to do with it.

Then there is the address book nobody set up. Every client keeps a list of addresses you have typed, separate from Contacts and invisible until it disappears. It is a client file. It does not travel. It is in no export. Its absence gets noticed a week later.

What each route costs you

Read the row for the pair you are moving between.

The pattern is the same on every row. The server side is free and the local side is the work. That local side is a file in a format the destination cannot read, which is where a converter earns its place.

One row deserves singling out. Legacy Outlook for Mac reaches end of support in October 2026 and the replacement cannot import what the old one exports, which makes it the one switch with a deadline attached.

What to do before you add the new account

Four things, in this order. The order matters.

The export step has its own limits worth knowing in advance, because the built in export leaves things behind that people assume are included.

What actually goes wrong

Almost every failed email migration is one of three things.

The first is local mail that nobody checked for. It stays invisible in the new client. It stays invisible in the old one once that goes. By the time anybody misses it the machine has gone.

The second is a folder tree that arrives flat. Nesting in a local mail file comes from how the files sit in directories rather than from anything inside them. A naive import then produces one long list where a hierarchy used to be.

The third is assuming the new client will ask. It will not. It adds the account, syncs what the server offers and reports success, because from its point of view nothing is missing.

Which client writes what decides how much you can take with you, which is a question about storage formats rather than features.