With WebSphere MQ, where is a message addressed to a remote queue manager and PUT under syncpoint control stored until MQCMIT is issued?
Messages that are addressed to a remote queue manager resolve to a transmit queue. Which transmit queue they resolve to depends on how the message will be sent to the remote QMgr. The message will resolve locally to either a user-defined transmit queue for a SDR or SVR channel, or it will resolve to SYSTEM.CLUSTER.TRANSMIT.QUEUE for a clustered channel.
For any message that is put under syncpoint, the message is written to the transaction logs and, if the message is persistent, to the queue file. The queue depth increases to reflect that the message is in the queue but the message is not made available to other programs (such as the channel agent) until the COMMIT occurs.
So if your message is going to a clustered queue manager and you PUT under syncpoint, you will see the depth of the cluster transmit queue increase. At that point the message is at least in the transaction log and possibly also written to the queue file. As soon as the message is committed, it becomes available to the channel.