Methods, systems and computer program products for operating an output processor a transaction processing system are provided. Aspects include receiving a request by an output processor to deliver an output message having a plurality of message segments and obtaining a target buffer size. Aspects also include allocating an output buffer for the output message, the output buffer having the target buffer size and iteratively obtaining a message segment of the plurality of message segments and storing the message segment in the output buffer. Based on a determination that all of the plurality of message segments have been stored, aspects include delivering the output message. Based on a determination that the output buffer is full and that all of the plurality of message segments have not been stored in the output buffer, aspects further include increasing the target buffer size to a maximum buffer size.