Going the direction nobody writes guides for: leaving Outlook for Thunderbird permanently. One IMAP account plus a 15GB PST archive of old mail.
The account part seems obvious but how does the PST archive make the trip?
Move emails from Outlook to Thunderbird
Solved Email & Outlook
BA
Barry Allen May 12, 2021
2 replies
10,650 views
Reviewed by moderators
Right instinct on both counts. The IMAP account just gets added to Thunderbird and syncs itself, the PST is the actual migration and Thunderbird cannot open it, today or ever, so it needs converting to something Thunderbird reads natively.
The direct route: convert the PST to EML or MBOX with a converter, SysTools PST Converter outputs both, then import into Thunderbird's Local Folders with the ImportExportTools NG add-on. MBOX imports arrive one folder per file, EML imports recreate a directory tree, either preserves your archive structure. Fifteen GB converts locally in an hour or so, no servers involved.
The free route: while Outlook still works, drag folders from the attached PST into the IMAP account and let them upload, then Thunderbird downloads them on its side. Zero tools, but 15GB through a mail server takes days, eats server quota and anything above the provider's size limits bounces. Fine for a slim archive, painful for yours.
Whichever route, keep the original PST untouched until Thunderbird shows matching folder counts. And export contacts and calendar separately before decommissioning Outlook, CSV for the address book and ICS for the calendar, since neither rides along with mail in any conversion.
Converted to MBOX and imported the archive locally in about 90 minutes total, structure intact. The account synced itself as predicted. Fully moved in a day.