Convert Thunderbird IMAP mail folder to maildir

I currently use mu4e with a maildir storage format as my email client. I used to use Thunderbird with an old Outlook based email account for work, using IMAP. In my .thunderbird directory tree, I can still find a directory with files called INBOX, Sent-1 etc. where the files appear to be concatenated email messages corresponding to a folder with the corresponding filename. There are also corresponding .msf files for (most) of the email files in the same directory. I want to convert this directory and its files into the maildir format, for ease of use. There are a bunch of tools out there for converting mbox formats in Thunderbird to maildir, including the [perl script mb2md](http://batleth.sapienti-sat.org/projects/mb2md/) (which I believe is derived/related to [this Python script](https://www.gerg.ca/hacks/mb2md/)) . I've tried this out and get the error

Fatal: Source is not an mbox file or a directory!

Are my files in the mbox format? They don't have the appropriate extension (i.e. .mbox) but as I understood it the format is just a single file per mail folder as I've described above. Or is there something else I have to do? **EDIT** This is the command that generates the error. I have not created the maildir_outlook directory, but I don't think that's the problem.

perl mb2md-3.20.pl -s outlook.office365.com/INBOX -d maildir_outlook

For clarity, the INBOX file looks like this (after some anonymization):

From
Received: from ***.OUTLOOK.COM by ***.OUTLOOK.COM with HTTPS via ***.OUTLOOK.COM with HTTPS via *** Fri, 11 Oct 2019 08:04:18 +000
Received: from *** ...
Received: from *** ...
Authentication-Results: spf=pass (sender IP is ***.***.***.***) 
smpt.mailfrom=*** ...
Received-SPF: Pass ...
Received: from ...
Received: from ...
Received: from ...
...
    MESSAGE BODY

and this just repeats with all the emails contained in the INBOX folder. The result of file INBOX is:

INBOX: ASCII text, with very long lines (678), with CRLF line terminators