IMSS does not deliver any mails to the Exchange Server. For each email, you will see the following entry in the message tracking of the console:
452 4.3.1 Insufficient system resources\r\n
This error happens because the Exchange Server is either unavailable or has low disk space.
To resolve the issue, check and free up the disk space on your Exchange Server. For more information, refer to this Microsoft article: Troubleshooting the "452 4.3.1 Out of memory" error message in Exchange Server 2003 and in Exchange 2000 Server.