This indicates an attack attempt to exploit a Buffer Overflow Vulnerability in Exim.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when parsing crafted SMTP requests. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted SMTP request.
Exim prior to 4.90.1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply patch available from the vendor.