This indicates an attack attempt to exploit a remote Command Injection vulnerability in SendMail in which was exploited by Morris Worm back in 1988.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. An authenticate remote attacker may be able to exploit this to execute arbitrary OS commands within the context of the application, via a crafted SMTP request.
Sendmail version 5.60 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
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