  Music recommender systems have become an integral part of music streaming
services such as Spotify and Last.fm to assist users navigating the extensive
music collections offered by them. However, while music listeners interested in
mainstream music are traditionally served well by music recommender systems,
users interested in music beyond the mainstream (i.e., non-popular music)
rarely receive relevant recommendations. In this paper, we study the
characteristics of beyond-mainstream music and music listeners and analyze to
what extent these characteristics impact the quality of music recommendations
provided. Therefore, we create a novel dataset consisting of Last.fm listening
histories of several thousand beyond-mainstream music listeners, which we
enrich with additional metadata describing music tracks and music listeners.
Our analysis of this dataset shows four subgroups within the group of
beyond-mainstream music listeners that differ not only with respect to their
preferred music but also with their demographic characteristics. Furthermore,
we evaluate the quality of music recommendations that these subgroups are
provided with four different recommendation algorithms where we find
significant differences between the groups. Specifically, our results show a
positive correlation between a subgroup's openness towards music listened to by
members of other subgroups and recommendation accuracy. We believe that our
findings provide valuable insights for developing improved user models and
recommendation approaches to better serve beyond-mainstream music listeners.
