1. A method of routing messages, comprising: receiving, at a routing device, a request message from an originating device to be forwarded to one of a plurality of target devices, the request message having a first network address as a source address identifying the originating device; dynamically mapping the first network address of the request message to a second network address of a selected target device of the plurality of target devices, storing the first network address and second network address in association with each other as address mapping information, and forwarding the request message to the selected target device using the second network address; receiving from the selected target device an error message in relation to the request message; identifying the originating device which originated the request message using the address mapping information and the second network address of the selected target device which issued the error message; and configuring the routing of a later request message from the identified originating device, wherein configuring the routing comprises configuring the routing device to not to forward the later request message received from the identified originating device to any of the plurality of target devices; updating the address mapping information to indicate that the first network address should be used when transmitting to the selected target device subsequent messages received from the originating device; updating the address mapping information to associate the first network address with a third static network address of the selected target device to which the subsequent messages from the originating device should be sent, in response to receipt of the error message; receiving at the routing device a second request message following the request message which triggered the error message from the originating device, and forwarding the second request message to a server at the third static network address, receiving from the selected target device a second error message in relation to the second request message; and selecting at the routing device a target device to which the request message is to be sent based on a workload balancing algorithm.