Random emails deferred - Command died with signal 11: "/usr/lib64/plesk-9.0/postfix-local"

I searched the internet for days and examined the server even more days, but I can't get a cause and therefore a resolution for this problem. It's not mentioned in any Google search results, only in just a handfull as seperate parts (signal 11 which is some kind of segmentation fault or memory problem, or Command died with signal xxx: "/usr/lib64/plesk-9.0/postfix-local" where xxxx is not 11) that do not seem to be related in any way to my problem.

Random email get stuck on the server,...

Random emails deferred - Command died with signal 11: "/usr/lib64/plesk-9.0/postfix-local"