Adrian Goenendyk wrote
If Groups.io cannot accommodate you, you should be able to do this yourself with two pieces of software from Personal Groupware.
Personal Groupware Offline (PGO4) automatically downloads all the messages (and attachments, files, and photos if you have them.
Personal Groupware Offline to Groups,io (PGO-2-GIO) uploads the files to a group you create in Groups.io. Message numbers will be maintained, and attachments (if any) will be retained.
At this very moment I just checked and downloads from Yahoo are working.