When reviewing the Messaging Gateway mail logs a number of WARNING level messages have been logged indicating that the mail server is waiting for the spool load to complete.
You are receiving Warning messages such as the ones below:
Sun Sep 5 10:00:11 2016: WARNING: waiting for completion of spool load: 16810 / 71720 messages.
Sun Sep 5 10:01:14 2016: WARNING: waiting for completion of spool load: 33475 / 71720 messages.
Sun Sep 5 10:02:17 2016: WARNING: waiting for completion of spool load: 52107 / 71720 messages.
Messaging Gateway
These messages are logged during the mail server (MTA) process startup and the MTA is loading the message queues into memory. Usually this phase of the startup complete quickly but when there are a large number of messages in the delivery queue it can take a few minutes to fully process and load the queue information into memory.
The message spool will load by itself and does not require any user action.
If you are not seeing any mail flow delays and your server is processing a high amount of Email, then these messages can be safely ignored.