Hi Ashraf,

It's not an error; it's just an alert notification sent to the admin because the "The combined message queue is larger than 1GB" Disk Space alert is enabled by default on Administration > Alerts page.  This alert indicates that the total size of all the queues (inbound, outbound and delivery queues) is more than 1GB. 

You can run the "mta-stats" command to check the sizes of individual queues. 

Here is the output from my lab box (it is not processing any mail, so the size of all queues is 0):

vm-sflabga> mta-stats
delivery_connections: 0
delivery_data_rate: 0 bytes/sec
delivery_deferred_msgs: 0
delivery_msg_rate: 0 msg/sec
delivery_queue_size: 0 KB
delivery_queued_msgs: 0
inbound_deferred_msgs: 0
inbound_listener_connections: 0
inbound_listener_data_rate: 0 bytes/sec
inbound_listener_msg_rate: 0 msg/sec
inbound_queue_size: 0 KB
inbound_queued_msgs: 0
outbound_deferred_msgs: 0
outbound_listener_connections: 0
outbound_listener_data_rate: 0 bytes/sec
outbound_listener_msg_rate: 0 msg/sec
outbound_queue_size: 0 KB
outbound_queued_msgs: 0 

Regards,

Adnan