What is the correct value within the field Application server service (Gateway) for sender channels?
This value has to be the name of the service port which is running the gateway. Normally this will be a name like sapgwXX, where XX is the system number of the particular system. This value also can be looked up in the gateway monitor. Open transaction SMGW and choose Goto -> Parameters -> Display. Beneath Attributes there will be the entries ‘gateway hostname’ and ‘gateway service’.