Mail in Apple Mac OS X before 10.6.3 does not disable the filter rules associated with a deleted mail account, which has unspecified impact and attack vectors.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2010-0539
http://support.apple.com/kb/HT4077
http://lists.apple.com/archives/security-announce/2010//Mar/msg00001.html