The subject message happens when the Internet Mail Service (IMS) detects low disk space on the drive that holds the spool directories. If the available space drops belows the threshold (10MB default), IMS prevents new messages from the internet and the MTS-OUT queue (NOT the SMTP service or content conversion algorithms). IMS checks the threshold every 15 seconds.
The messages you receive are:
SymbolicName=MSG_ENTERING_FLUSH_MODE EventType = Warning Category = Message Transfer LogLevel = 0 The available disk space on the spool drive has dropped below %1 kilobytes. The Internet Mail Connector will not accept messages for inbound and outbound conversion. Inbound and outbound message conversion will continue when the available disk space has increased above %1 kilobytes.When available space then rises above the threshold:
SymbolicName=MSG_LEAVING_FLUSH_MODE EventType = Warning Category = Message Transfer LogLevel = 0 The available disk space on the spool drive has increased above %1 kilobytes. The Internet Mail Connector is once again accepting messages for inbound and outbound conversion.You can modify the treshold by using Regedt32 to navigate to:
On the Edit menu, Add Value name DiskSpaceLowWaterMark, as a type REG_DWORD, and set the data
value to the number of bytes, using the Decimal Radix. If you want the threshold to be 25MB, enter 25600000
(25 * 1,024,000).