I am getting 4.3.1 Insufficient System Resources error when submitting large emails (>500kb with attachments) on Exchange server. Back pressure is disabled and no alerts come up in the Events (Microsoft Server 2008). There is >4gb disk space on the system drive and 120gb of the drive where queue database/mail database is located. I have recently moved mailbox database and data storage directory from the system drive to the secondary drive.
What can be causing this error?