How it works

Moving mail from an mbox archive into a PST file sounds like copying a document between folders. It isn't. The two formats were built on opposite ideas about what an email even is.

A PST file (Personal Storage Table) is Microsoft's own format, and it behaves less like a file and more like a small, live database. One PST holds a whole structure: folders, subfolders, every message, and the categories you've put on them, all linked together in a tree. It also remembers things about each message that have nothing to do with the text, like whether you've read it, whether you've flagged it for follow-up, what category you gave it, retention rules, even digital signatures. It carries the kind of housekeeping that Outlook and Exchange accounts depend on.

An .mbox file is the opposite. It's a plain text file that stores messages one after another, and it only really knows about the email itself: the words, the headers, and any attachments. It records what the message looked like the moment it was saved, and nothing about how you had organised or marked it. It's a snapshot, not a living system.

That gap is the whole problem. PST expects a rich, organised world. Mbox hands over a flat list.

Mail apps know mbox is bare, so over the years each one bolted its own system on top. The catch is that usually only that app can read it back.

Thunderbird keeps your tags, your read and unread state, and your starred messages in a separate sidecar file with a .msf extension. That file is written in an old Mozilla format called Mork that is notoriously awkward to read. Thunderbird also fakes subfolders using .sbd directories that sit next to the mailbox.

Gmail does it differently again: export your mail with Takeout and everything lands in one big mbox, with your labels tucked inside each message as special headers likeX-Gmail-Labels.

Apple Mail goes its own way too. It stores each message as a separate.emlx file, which is the raw message with a small Apple plist tacked on the end. Your flags live in that plist as a single number, where each bit means something: one bit for read, another for flagged, another for replied. On top of that it keeps an Envelope Index, a SQLite database that caches all of this so search stays fast. Even the mailboxes are not what they look like, since a.mbox here is actually a folder rather than a single file.

So the same simple idea, "this email is tagged Work," ends up stored in a completely different place and shape depending on where the mail came from.

Pulling the actual email out of an mbox and writing it into a PST is the well understood part. The body, headers, and attachments all follow a public standard called MIME.

Even then there's a catch. Microsoft documents the PST format itself, but Outlook offers no built-in way to turn an mbox into a PST — so most tools cheat by remote-controlling an installed copy of Outlook, which must be licensed and sits locked for the whole run. We didn't want that, so ContinuMail builds each PST from scratch with its own engine, writing the file's internal database structure directly. No Outlook involved, before, during or after.

The genuinely hard part is everything that isn't the message: the flags, the tags, the folder shapes, and the app-specific extras. Each one has to be found in its hiding place, decoded from a format that was never meant to be shared, and then translated into the matching Outlook idea. A Thunderbird tag becomes an Outlook category. A starred message becomes a follow-up flag. None of these are straight copies. Each one is a small translation, and getting them all right is most of the work.

The result is an ordinary Outlook data file. Open it in Outlook and your folders, your messages, and the marks you put on them are simply there — and if you converted a Thunderbird profile, so are your calendars, contacts and tasks — with no Outlook needed to create the file and nothing uploaded anywhere. When you're ready, thestep-by-step guidewalks through the actual conversion. Moving specifically from Thunderbird? TheThunderbird to Outlook guidecovers the profile scan and how your tags become Outlook categories.