I must say, problem is resolved with today's update of linux image. linux-image-3.2.0-4-rt-amd64 3.2.53-2 proposed updates -- Claws Mail version 3.8.1 Linux debian 3.2.0-4-rt-amd64 #1 SMP PREEMPT RT Debian 3.2.53-2 x86_64 GNU/Linux