What does the error message "452 4.3.1 Insufficient system resources" mean?

This error message is generated by Exchange and just relayed by POPcon. With this SMTP error message Exchange is telling us that no new emails are accepted because the harddisk space is less than 4GB.

You need to clear disc space for Exchange to accept new emails and then move the Emails that POPcon moved into the BADMAIL folder as *.msg files into the PICKUP folder (both in the c:\program files\X86\POPcon folder) to have them resent to Exchange.

This is a MS TechNet article on the Exchange feature: Back Pressure.