Handle the format as used by Listserv.[1]
http://blog.anthonyrthompson.com/2010/06/listserv-to-mailman-converting-listserv-archives-to-mailman/
http://www.hypermail-project.org/archive/99/0520.html (copy of script)
This should work, the old Hypermail ls2mail circa 2004.
[1] In some configuration? The format is not using 'From ' separators.
Actually the script(s) doesn't split in all the right places. Thus the code needs to be adapted. Cost A: $50
Add the code to ml2json itself so that Listserv style files are detected and handled automatically. Cost B: A+$50
XXX
Alternative split script:
http://verify.rwth-aachen.de/psk/ls2mm/lsa2mma.py
⇒ Cost range: $50..100