Dovecot 1.2.x before 1.2.15 allows remote authenticated users to cause a denial of service (master process outage) by simultaneously disconnecting many (1) IMAP or (2) POP3 sessions.
http://www.vupen.com/english/advisories/2011/0301
http://www.vupen.com/english/advisories/2010/2840
http://www.ubuntu.com/usn/USN-1059-1
http://www.redhat.com/support/errata/RHSA-2011-0600.html
http://www.mandriva.com/security/advisories?name=MDVSA-2010:217
http://www.dovecot.org/list/dovecot/2010-October/053450.html