I did the same sometimes ago. I had a very large number of messages so I had to break things down into separate mbox according to years and months. I then just

I did the same sometimes ago. I had a very large number of messages so I had to break things down into separate mbox according to years and months. I then just loaded the file into the email client. I wanted to sync the mailbox with Gmail (that would be closest to getting it back online) but never had the time. However, I wish there was an exporter for Google Groups but there isn’t any as far as I could tell.