The Data function in class.smtp.php in PHPMailer 1.7.2 and earlier allows remote attackers to cause a denial of service (infinite loop leading to memory and CPU consumption) via a long header field.
http://www.vupen.com/english/advisories/2007/2242
http://www.vupen.com/english/advisories/2006/0448
http://www.securityfocus.com/bid/13805
http://sourceforge.net/project/shownotes.php?release_id=341210&group_id=26031
http://secunia.com/advisories/25726
http://secunia.com/advisories/18732