Moving to MailCode from another mail client

Apple Mail · Outlook · Thunderbird · eM Client · Spark · Mailspring · MailMate · Postbox

Most of a switch is not a migration. If your account is Exchange, Microsoft 365 or IMAP, the mail is on the server, and adding the account to MailCode brings all of it down — there is nothing to export. Importing matters only for the mail that exists only on your disk: a POP account, an On My Mac mailbox, a Local Folder, an old PST. This page separates the two, client by client, and says plainly where MailCode cannot read a format.

First work out which kind of mail you have

Open your current client and look at where a mailbox sits in its sidebar.

Under an account name (an email address)

Server mail. Add the same account in MailCode and it syncs down — Exchange EWS, Microsoft 365, or plain IMAP. Nothing to export, and nothing you can lose by trying, because reading a mailbox does not change it.

Under On My Mac, Local Folders, Archive, or a file you opened by hand

Local mail. It exists in your old client's own storage and nowhere else, so it has to be exported into a format MailCode reads. That is the rest of this page.

What MailCode reads

Format	Read	Usually comes from
.pst / .ost	Yes, directly	Outlook for Windows
.eml	Yes, single files or whole trees	Almost every client, by drag or export
.msg	Yes	Outlook, saved one message at a time
maildir	Yes, including cur and new	Unix servers, some power clients
mbox	Yes, each mailbox as a folder	Apple Mail export, Thunderbird, Postbox
.emlx	Yes, from Apple Mail's library folder	Apple Mail's own storage
winmail.dat	Yes, opened from the message it arrived on	Outlook, when it sends Rich Text mail
.olm	No	Outlook for Mac

All of these come in through the same command, File → Index EML Files…: point it at a file or a folder and it finds the mail inside, subfolders included. The one format it does not read is .olm. For that, select the messages in Outlook for Mac and drag them to a folder in Finder, which writes one .eml per message, then point MailCode at the folder.

Apple Mail (Mac Mail)

Apple Mail keeps its mail as .emlx files inside ~/Library/Mail, and exports a mailbox as an .mbox. MailCode reads both.

• IMAP, Exchange, iCloud, Gmail or Microsoft 365 accounts: add the account in MailCode. The mail comes down from the server. Nothing to export.
• On My Mac mailboxes and POP accounts: in Apple Mail, select the mailboxes and choose Mailbox → Export Mailbox…. In MailCode, choose File → Index EML Files… and pick the folder you exported into. Each mailbox lands as a folder named after it, with its messages and attachments.

To skip the export, point the same command at ~/Library/Mail and MailCode reads the .emlx files directly. One limit: when Apple Mail has stored a message's attachments separately (a .partial.emlx), the message and its text come across without those attachments.

Outlook for Windows

This is the easiest of all of them, because Outlook's own file format is the one MailCode reads directly — there is a longer walk-through at how to open a PST file on a Mac.

• An existing PST: look in Documents\Outlook Files\. Point MailCode's PST import at the file. Nothing has to be exported first, and Outlook does not need to be installed.
• An OST — the cached copy of a server mailbox, in %LOCALAPPDATA%\Microsoft\Outlook\ — is the same on-disk format, and the importer reads it as it stands. Copy it somewhere else first; Outlook holds a lock on the live one.
• A mailbox you want to freeze as a file: File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst).

The file's own folder tree comes across as it stands — Inbox, Sent Items, the subfolders you made in 2014 — into whichever destination folder you pick. Attached messages, and bodies that exist only as HTML or RTF, come with it.

Outlook for Mac

Outlook for Mac exports .olm, not .pst, and MailCode does not read .olm. There are two ways around it:

• Select the messages in Outlook and drag them to a Finder folder — you get .eml files, and MailCode imports the folder.
• If the same mailbox is still on Exchange or Microsoft 365, skip the export: add the account to MailCode and let it sync.

Thunderbird

Thunderbird stores each folder as one mbox file, and MailCode reads those directly. Quit Thunderbird, then choose File → Index EML Files… in MailCode and pick the profile's Mail/Local Folders folder (Thunderbird's Help → Troubleshooting Information → Profile Folder shows where it is). Each Thunderbird folder becomes a folder in the archive, subfolders included, rather than everything landing in one heap.

eM Client

eM Client exports .eml natively: File → Export → Export mail to .eml files. Choose a folder, then import that folder in MailCode. Folder structure is preserved on both sides.

Spark, Mailspring and other IMAP front-ends

These keep no archive of their own that outlives the account — they are windows onto an IMAP or Exchange mailbox, with a local cache underneath. There is nothing to export. Add the account in MailCode and the mail arrives.

The thing to check before you delete the old client is whether it was doing anything to the server your new one has to keep doing: a Spark shared inbox, or a send-later queue that has not fired yet.

MailMate

MailMate is IMAP-backed, so the account route applies. For anything held locally, dragging a selection to Finder gives .eml files, and MailMate's own Export Messages… can write the same. Its mbox output is read too.

Postbox

Postbox keeps its mail in mbox files the way Thunderbird does, so the Thunderbird route applies: quit Postbox, then point File → Index EML Files… at its profile's mail folder.

Indexed where it sits, or copied in

When you import, MailCode asks one question worth understanding, because it is hard to change your mind about later:

Indexed (the default)

The archive keeps the index and reads each message back from its file. Fast, small, and the folder has to stay connected — which is the right answer for an external SSD full of old mail.

Copied into the database

Bodies and attachments are copied into the archive and the files are no longer needed. Fine for a few thousand messages; a very large archive imported this way makes the database enormous.

One folder holds one kind or the other, never both. Re-running an import over the same folder is safe — files already in the archive are skipped, so a second pass adds what is new instead of duplicating what is there.

What is actually different once you are across

Everything below follows from one decision: the mail lives on your disk, in a full-text index, as it arrives.

• Search that answers while you type. Measured at 4–48 ms (median to 95th percentile) over a real 285,978-message, 31.8 GB archive, searched across seven mailboxes at once.
• An archive that outlives the mailbox. A mailbox has a size limit and a retention policy; a disk has neither. MailCode can move mail off the server deliberately, fetching every attachment byte before the server copy goes.
• Old formats stop being a dead end. The PST from the project that closed is searchable in the same box as this morning's mail.
• Mail you can act on. Flags with due dates, notes with or without a message, scheduled send that survives quitting the app, and rules that will not undo a decision you made by hand.
• Translation in place, on the message you are looking at, attachments included.
• Nothing of ours in the middle. No account to create with us, no analytics in the application, and AI features off until you turn one on.

What you give up

Worth knowing before you move, not after.

• No mobile app. MailCode is a desktop client. Your phone keeps talking to the same server.
• macOS 12 or later, or Windows 10 and 11. Mail you import on one computer stays on that computer's disk.
• No shared team inboxes or built-in chat. If that is why you use Spark or eM Client, this does not replace it.
• .olm is not read directly. Outlook for Mac mail comes across by dragging it to Finder first, as above.

The order to do it in

1. Add your accounts. Let the first sync finish.
2. Import the PSTs — those need no conversion.
3. Export anything local from the old client as .eml, keeping its folder structure.
4. Import that folder, with Keep the folder structure on disk left on.
5. Run a duplicate scan. Years of imports and re-syncs leave copies, and one pass across the whole archive marks them in the list.
6. Keep the old client installed until you have searched for something you know is there and found it.

Step six is not a formality. It costs nothing and it is the only honest way to know the move worked.