Method of re-routing mail traffic exchanged with a messaging system in a data transmission system in which at least one user connected to an IP transport network such as the Internet can transmit requests to and receive data from a plurality of servers through a load balancer, wherein at least one of the servers being a mail server in charge of mail exchange with the user through the IP transport network. This method comprises the steps of forwarding a routing context to an external service infrastructure partner (SIP) device when the system is triggered by an event A, reconfiguring the load balancer in order to be able to send a mail protocol to the SIP device when receiving the mail traffic to be re-routed to the SIP device, and forwarding a user context to the SIP device.