He can choose to get notifications from this category through email without a problem.
I’d say first, we should figure out why he cannot log in (he says on-list that he gets an “Account cannot be found” for some reason") - I’ve replied on-list with his username, maybe that’ll help him to log in.
Once he can actually get emails from our system, we can figure out how best to get his notifications working. He can easily just set only notifications from all posts in the “digiKam” category (here basically).