In diffusive molecular communications (DMC), the relay serves to address the challenge in long-distance DMC scenarios. Currently, the relay used in DMC mainly includes the amplify-and-forward relay and decode-and-forward relay. To facilitate information transmission, both of these relays are required to actively release information molecules, while consuming the energy stored by themselves and introducing a transmission delay. In this paper, a passive relay is proposed to support long-distance communications without excess energy consumption. Specifically, we consider a multi-user communication system with both near and far users, where it is assumed that specific types of molecules serve designated users and are reflected by other users during propagation. Due to this molecule reflection, the near user can function as a passive relay, aiding in long-distance transmission for the far user. Besides, two detectors—namely, the ideal and actual maximum likelihood detectors—are designed for the proposed system, and their detection performance is analyzed respectively. Moreover, energy consumption is studied for the proposed scheme. Numerical results indicate that the bit error rate (BER) performance can be greatly improved at a low synthesis cost.