Geary before 0.6.3 does not present the user with a warning when a TLS certificate error is detected, which makes it easier for remote attackers to conduct man-in-the-middle attacks via a crafted certificate.
https://git.gnome.org/browse/geary/commit/?h=geary-0.6&id=55f06a7bdcedb7efde6a516bde626ea28793ca7e
https://bugzilla.gnome.org/show_bug.cgi?id=713247
http://lists.opensuse.org/opensuse-updates/2014-09/msg00035.html