A transaction processing and authorization service. A transaction initiation message received from a source entity by the service may include one or more errors. The service may, in response to receiving an incorrect transaction initiation message, contact the source entity via a second communications channel and allow the source entity to correct the faulty transaction initiation message during the communication session. The service may allow the source entity to correct only incorrect or missing portions of the transaction initiation message, and thus the source entity may not be required to re-enter the entire message. Correction of the transaction initiation message may be performed as part of an authorization contact from the service to the source entity on a communications channel, and thus the correction may not require any extra communications attempts or sessions between the service and the source entity.