Bacteriophages, viruses infecting bacteria, recognise their host with high specificity, either binding to 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 the majority of bacteriophages. Here, we present the electron cryo-microscopy structure of bacteriophage T5 RBPpb5 in complex with its E. coli receptor, the iron ferrichrome transporter FhuA. Monomeric RBPpb5 is located at the extremity of T5 long flexible tail, and its irreversible binding to FhuA commits T5 to infection. Analysis of RBPpb5 structure within the complex, comparison with its AlphaFold2 predicted structure, and its fit into a previously determined map of T5 tail tip in complex with FhuA allow us to propose a mechanism of transmission of RBPpb5 receptor binding to the straight fibre, initiating the cascade of events that commits T5 to DNA ejection.