Bacteriophages, viruses infecting bacteria, recognize their host with high specificity, binding to either saccharide motifs or proteins of the cell wall of their host. In the majority of bacteriophages, this host recognition is performed by receptor binding proteins (RBPs) located at the extremity of a tail. Interaction between the RBPs and the host is the trigger for bacteriophage infection, but the molecular details of the mechanisms are unknown for most bacteriophages. Here, we present the electron cryomicroscopy (cryo-EM) structure of bacteriophage T5 RBP$_{pb5}$ in complex with its Escherichia coli receptor, the iron ferrichrome transporter FhuA. Monomeric RBP$_{pb5}$ is located at the extremity of T5’s long flexible tail, and its irreversible binding to FhuA commits T5 to infection. Analysis of the structure of RBP$_{pb5}$ within the complex, comparison with its AlphaFold2-predicted structure, and its fit into a previously determined map of the T5 tail tip in complex with FhuA allow us to propose a mechanism of transmission of the RBP$_{pb5}$receptor binding to the straight fiber, initiating the cascade of events that commits T5 to DNA ejection.