source: https://www.securityfocus.com/bid/13805/info

PHPMailer is affected by a remote denial-of-service vulnerability.

An attacker can send an email message with a malformed header field to initiate an infinite loop in the application. This eventually leads to a crash due to resource exhaustion.

PHPMailer 1.72 and prior versions are affected by this issue. 

From: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA... x 998