This indicates an attack attempt to exploit an Information Disclosure vulnerability in PHPMailer.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted request. A remote attacker may be able to exploit this to access sensitives data, via a crafted request.
PHPMailer prior to 5.2.22
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems
Apply the most recent upgrade or patch from the vendor.