Have you received other reports about problems like these in Claws Mail? Most recently we at Fedora have had https://bugzilla.redhat.com/702120 with 3.7.9, where the corruption caused g_strdup to crash while trying to alloc 11 bytes. In other reports (e.g. http://bugz.fedoraproject.org/claws-mail lists several 'malloc' tickets), the malloc crash affected various parts of the code. These issues are tough ones, because the backtraces only point at the side-effect, and not even the bug reporters are able to reproduce the crashes. They occur rarely (and probably too rarely to run in valgrind), but so far, a common pattern seems to be that the network connection has been lost/switched, and sometimes that suspend/resume has been used.
Closing in favour of bug 2769 and due to the comments in bug 3084.