[Discuss] Migrating hundreds of mbox files to an IMAP server?

[admin at bclug.ca (Ron)]

Daniel Barrett wrote on 2025-06-01 15:35:

> What's the best way to import 600 mbox files to an IMAP server?

Ouch, interesting question with many rabbit holes.


> Background: The files were created locally by Thunderbird, arranged in
> a file hierarchy. I'd like the same hierarchy to be created on the
> IMAP server. For example, the mbox file named Recipes.mbox would
> migrate to a new IMAP folder named Recipes, and the mbox file at path
> Foo/Bar/Blat.mbox would migrate to a new IMAP folder "Blat" within
> folder "Foo/Bar" on the IMAP server.

There are many ways an IMAP server can store messages: mbox, MailDir, 
MailDir with LAYOUT=fs (my favourite).


> I don't have direct filesystem access to the IMAP server. (It's
> fastmail.com.)

That makes it much easier - use Thunderbird to copy everything between 2 
accounts and let whichever system the IMAP server is using handle the 
details.


Side note, Thunderbird supports MailDir storage locally (each message is 
its own file instead of each folder being a large file with many messages).

Any corruption to a single message doesn't cascade to subsequent messages.


The TB docs say it's not recommended, however I've got many accounts 
using it for the past few years without any issues what-so-ever.

Some maildir "features" aren't supported by TB, but they are irrelevant 
(i.e. renaming the file to indicate it has been read or in some way has 
had its status changed). TB takes care of those elsewhere.

This TB Maildir setting must be turned on *before* accounts using it are 
created. Existing accounts will continue with mbox format.