WebShield SMTP 4.5 allows remote attackers to cause a denial of service by sending e-mail with a From: address that has a . (period) at the end, which causes WebShield to continuously send itself copies of the e-mail.
https://exchange.xforce.ibmcloud.com/vulnerabilities/5100
http://archives.neohapsis.com/archives/ntbugtraq/2000-q3/0101.html