So I succeeded in importing 20 years of mbox archives of an existing mailing list thanks to the good instructions on this list. Over the time period in question there are more than 6.5K unique users. They have been created as staged users, not yet activated.
My question is, for current users of that list, how do they activate their accounts? As far as I can see they cannot log in and so they can’t activate. Do I have to manually activate hundreds of individual users? What to do?
That looks to me exactly like having to do it individually for hundreds of users. Not the answer I was looking for. I may as well go through the last couple of years of posts in the web interface and manually activate.
What do other people do regarding large mbox archive imports then?
Your users need to sign-up at your new Discourse site with the same email address they used on the mailing list. Their staged user will be converted into a regular user during the sign-up.
20 years is a long time, so I assume some of your users might have used a bunch of different email addresses in all those years. I suggest they sign up with their current email address and you can manually merge the other staged users into that account.