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

[dbarrett at blazemonger.com (Daniel Barrett)]

On June 3, 2025, I wrote wrote:
>Thank you to everyone who responded to my question about importing
>large numbers of mbox files to an IMAP server. Here's a summary of
>what I learned from you.
>[description deleted]

FYI, the migration succeeded using mb2md to convert from mbox to
maildir, and mbsync to upload the messages to an IMAP server.  79GB of
email later, it's all uploaded to fastmail.com. The client (my wife)
is happy. :)

I ran into IMAP timeouts during the mbsync phase. It turned out that
the version of mbsync (isync) supplied with Ubuntu 22.04 LTS is old.
Downloading the latest isync from SourceForge solved the issue.

Next up: migrating another 100GB of email from Office 365 to Fastmail.
I'm probably preaching to the choir here, but this kind of migration
lays bare a steaming pile of bad email software. Every
Exchange-compatible email client I've tried either crashes, copies
only partially, or doesn't support email export.

The best approach so far is running a legacy version of Outlook in a
Windows10 VM to export PST files (making sure to disable "cached
exchange mode," or the PSTs are created empty), then Linux "readpst"
to convert to mbox, then the familiar mb2md and mbsync to finish the
job.  One folder at a time. Ugh! If anyone has a better method, please
offer it....

Dan