This indicates an attack attempt to exploit a Command Injection Vulnerability in a PHP IMAP module.
The vulnerability is due to an error while the vulnerable module handle a crafted mailbox string. A remote attacker may be able to exploit this to execute arbitrary commands within the context of the application, via uploading a crafted mailbox string in a HTTP request.
PHP version 7.0.30
PHP version 5.6.38 and prior
System Compromise: Authenticated attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: