On an active Messaging Gateway system, deleting large numbers of messages from the delivery queue via the command line mta-control utility may cause the mail server process to stop accepting and delivering messages until the service is restarted. While the mail server process is hung, the mta-control command also does not finish as expected.
Deleting large numbers of messages from the queue on a SMG system that is actively accepting and delivering messages can cause a deadlock in the scheduler. Accepting and delivering messages is waiting for the delete to complete and the delete is waiting on the acceptance or delivery of a message to complete.
This is not a common occurance and has only been observed on very active systems when upwards of 1000 messages are being deleted at a time.
This is a known issue and will be addressed with the SMG 10.6.5 release.