What gets converted
By Aksel VisbyPublished Updated Tested with ContinuMail Converter v0.3.2
A conversion tool asking for your mail archive owes you a straight answer to one question: what survives, and what doesn't? Here is that answer — including the parts that come across only approximately, and the parts that don't come across at all, with the reason why.
This page is the plain-language summary. The exhaustive version — every field, down to the MAPI property it lands in — is maintained inCONVERSION-FIDELITY.mdalongside the code.
From any mbox archive — Gmail Takeout, exported Local Folders, old POP stores — or a whole Thunderbird profile.
- Subject, From/To/Cc/Bcc (with correct recipient types), sent & received dates
- HTML bodies exactly as they were, with a plain-text version alongside
- Attachments — regular files, inline images (hidden, so no phantom paperclip), embedded .eml messages
- Message-ID and reply threading, importance/priority
- Folder structure, nested subfolders included — mirrored or flattened, your choice
- Read/unread state (from Thunderbird's index, Gmail's exported labels, or mbox status headers)
Comes across, with a caveat
- Plain-text version of an HTML-only message. Generated by a lightweight HTML stripper — the real HTML is what Outlook shows.
Extra fidelity from a Thunderbird profile
Thunderbird keeps per-message state in its .msf index, not the mbox. Point ContinuMail at a live profile and it reads that index directly.
- Authoritative read/unread, replied and forwarded marks
- Stars → Outlook follow-up flags
- Tags → Outlook categories, under your real tag names — colours baked into the PST
- Junk state — leave it, tag it as a Junk category, or move it to a Junk Email folder
- Message priority set inside Thunderbird
- Multiple accounts → one PST per account (or combined), each with its own calendars and contacts
- Messages Thunderbird deleted but hasn't compacted away are excluded, optionally
Comes across, with a caveat
- Bare exported .mbox without the profile. Only inline status headers are recoverable — read/replied/forwarded/starred where present; tags and junk state are not stored there.
- Category colours in classic Outlook. Colours are inside every PST, but classic Outlook only renders them when the PST is your primary/default data store; as a secondary file you see names without colours.
Calendar
Events from a Thunderbird profile's calendars become real Outlook appointments.
- Title, start/end with the original time zone, all-day events
- Recurring events — daily/weekly/monthly/yearly rules, deleted and modified occurrences
- Attendees with their accept/decline/tentative responses; the organizer field
- Reminders with lead time, location, free/busy status, private flag, importance, categories
- Event attachments (embedded), meeting identity for Exchange/M365-synced events
Comes across, with a caveat
- Teams / Google Meet links. Kept clickable in the event body — classic Outlook has no separate Join button to hang them on.
- Links between items (RELATED-TO). Preserved as a readable note in the body — Outlook has no native slot for them.
- Remote-URL attachments (e.g. a Drive link). Preserved as a link in the body — the converter never fetches from the network.
Not converted
- CalDAV sync bookkeeping (ETags, hrefs). Server-side state with no meaning inside a PST — dropped by design.
Contacts
Thunderbird address books — modern SQLite and legacy .mab both — become real Outlook contact cards.
- Names (display, first/middle/last), up to three email addresses
- Phone numbers mapped by type, home and business addresses
- Company, department, job title, websites, IM handles
- Birthday, anniversary, notes — and the contact photo
Not converted
- Distribution lists / groups. A planned follow-up.
Tasks
Thunderbird to-dos become Outlook tasks.
- Title, notes, start and due dates, completed state, percent complete
- Priority, categories, reminders, private flag
- Recurring tasks — Outlook regenerates the next occurrence when you complete one
Comes across, with a caveat
- A recurring task with deleted or overridden occurrences. Written as a single task with a warning rather than dropped.
Not converted
- Task assignees. A planned follow-up.
How this is validated
Every release runs a comprehensive automated test suite, a round-trip harness that reads the written PST back with an independent reader, and validation against real Thunderbird and Gmail Takeout archives — with Outlook's own repair tool (scanpst.exe) required to report a clean file before release. Keep backups and review the conversion report all the same: it's an early 0.x tool, and your mail deserves the caution.
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Ready to run one? Start with theThunderbird guide, the Gmail Takeout guide, or thegeneric mbox guide.