This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Exim.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a series of crafted SMTP packets.
Exim 4.89 and earlier versions
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.