Converting Thunderbird to Outlook (Office365/Outlook2016)

March 27, 2018

Featured, Reviews, Technology

For posterity, and the benefit of others considering the same thing, some quick notes about the exhauting experience of moving 20 years of email in Mozilla Thunderbird (52.6) to Outlook 2016 (and Office365)

Basic steps

  1. Setup your Office365 account
  2. Setup a new IMAP account for it in Thunderbird
  3. In TB, flatten your various subfolders into local one folder (say “All”) – new search tools (and good subject lines) mean you no longer need complicated subfolder structures
  4. Go to Offline Mode – this is important – it allows you to queue move in batches without waiting for the IMAP folder to update
  5. In TB, turn OFF Message Synchronising | Advanced (deselect all folders for Offline use) – this means messages wont sync back when you move them to the IMAP account
  6. Drag (move) messages from each POP account folder to the IMAP folder as follows
    1. In batches of less than 1000 for important/recent email
    2. In batches of 1001-5000 for less importants / older email
  7. Queue up to 20,000 emails ready for transfer to IMAP, and when ready, toggle back to Online mode – this will begin the long process of relocating your emails to be hosted on the IMAP server – this can take hours to days (I had nearly 200,000 messages)
  8. Open up Outlook and watch the magic as folders on your other Outlook account populate

Tips and Notes

  • I generally would not run both Thunderbird and Outlook at the same time – it seems to slow the proces
  • you may occassionally receive a message from Office365 that you’ve exceeded quota – wait a few minutes a retry
  • you may end up with up some duplicates in Outlook – can’t say if they existed before, but try DuplicateKiller Add-in $25


Although there’ve been some hiccups, the above approach will serve you well. Generally Thunderbird is hopeless at moving and/or managing massive email collections like mine. In the end, I moved about 12 GB of email using this method and, now that it’s all migrated to Outlook, I am very happy that all my email is quickly searchable.

Update #1

On second thoughts, after completing the above, I noticed a high level of duplicates, corrupted and empty-sender emails – to the extent that it has spooked me off this approach. I’ve revisted the management of this process and am currently trying the MBOX to PST route – the bought software $US29, seems quite good so far and, during testing, seems to retain the integrity of a range of message types/sizes/formats. There are a variety of products out there that manage this for you, but the TweakSoft MBOX to PST, whilst it seems the same as the variants, came recommended on TechNet.   So far so  good – 176,000 messages are presently being converted to a single PST, which I will attached as a data file in Outlook and then move to the online Archive folder.

Update #2

Products like MBOX to PST aren’t quick, or, more to the point, if you have a large email database, it will take some time to convert into a PST. Exporting to a MBOX file (which is the first step of converting to PST), is very quick (minutes), however converting a 140,000 message MBOX file can take upwards of two days (on four-core Core i7), after which, connecting the converted PST file as a data file and moving it to your master PST (mailbox) can take about an hour. So, in short, the process for large email collection is –> minutes + days + hour/s.

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About Justin Moss

An information and engineering technologist. Unabashedly opinionated, open-minded and an oft overthinker. Born Sydney Australia, home of the 'Southerly Buster'

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