End-point business system down-time

Now that we have a number of business scenarios in production, we are running into the situation where a particular end-point business system is being taken down for maintenance, but the XI system (as well as the sending business system) is not going out of service. The receiving business system is an R/3 system that is connected to via the IDoc adapter.

We would like to be able to take the R/3 system down and have XI somehow recognize that it is down and queue up any messages intended for the system, without generating an error. Has anyone dealt with a similar situation?

Regards,

Chris