When the convenience keyword :replies? is flagged, send-message-batch registers a mailbox per message to use as a reply repository.

This keyword currently defaults to t, since send-message-batch is almost everywhere paired with with-replies, which does the job of closing out the corresponding mailbox. On the other hand, this means that an inattentive programmer using send-message-batch away from with-replies runs a high risk of leaking resources.

This bit me recently, so I'm wondering if we should investigate this usability change.