SourceForge hosts mailing list, but can I export the messages to mbox format for migration?
You can export any SourceForge mailing list to 'mbox' format by visiting:
This requires authentication via SourceForge before you can proceed. More information at https://sourceforge.net/p/forge/documentation/Mailing%20List%20Archives/
Smaller mailing list archives tend to be easily downloadable from any web browser. Larger mailing list archives may timeout, but I've had success still downloading them via commandline tools like