smtp.c in the c-client library in University of Washington IMAP Toolkit 2007b allows remote SMTP servers to cause a denial of service (NULL pointer dereference and application crash) by responding to the QUIT command with a close of the TCP connection instead of the expected 221 response code.
https://exchange.xforce.ibmcloud.com/vulnerabilities/46604
http://www.securityfocus.com/bid/32280
http://www.openwall.com/lists/oss-security/2008/11/03/5
http://www.mandriva.com/security/advisories?name=MDVSA-2009:146