This paper examines the evolving role of preprint servers within the scholarly communication system and their relationship with peer-reviewed journals. Historically prominent in fields like physics and mathematics, preprints have flourished in popularity across various disciplines in the past decade. Today, over 50 preprint servers exist, offering a platform for early and rapid dissemination of research findings. Preprint servers have traditionally fulfilled several functions of scholarly communication, including registration of research findings and archiving. However, recent trends indicate a shift towards servers challenging the established roles of peer-reviewed journals by adopting aspects of the certification and curation function. Through qualitative analysis of in-depth interviews with 14 preprint server managers, this study explores how preprint servers view their role in the certification and curation of scientific knowledge, the significant challenges facing preprints, and the potential future dynamics between preprint servers and peer-reviewed journals. Our findings point to a lack of consensus on how the relationship between preprint servers and journals could or should develop, and to diverging views on how the certification and curation functions are best performed, and by whom. We consider these tensions in the context of the ongoing sustainability and credibility challenges faced by preprint servers, situate them within evolving research policy frameworks, and discuss their implications for the diversity of future scholarly communication practices.