Since 4.8.1 kmail from time to time just stalls. None of the UI buttons accept any clicks. It does not use any CPU, it does not crash, it is just dead forever.

I attached gdb to that process and got:
(gdb) bt
#0  0x00007fd0ecc73209 in syscall () from /lib64/libc.so.6
#1  0x00007fd0ee1a602b in _q_futex (val2=0, addr2=0x0, timeout=0x0, val=2, op=0, addr=0x7fd07c028d40) at thread/qmutex_unix.cpp:99
#2  QMutexPrivate::wait (this=0x7fd07c028d40, timeout=<optimized out>) at thread/qmutex_unix.cpp:113
#3  0x00007fd0ee1a1ded in QMutex::lockInternal (this=<optimized out>) at thread/qmutex.cpp:450
#4  0x00007fd0eacb5558 in lockInline (this=0x7fd07c0254a0) at ../../src/corelib/thread/qmutex.h:190
#5  QMutexLocker (m=0x7fd07c0254a0, this=<synthetic pointer>) at ../../src/corelib/thread/qmutex.h:109
#6  QDBusConnectionPrivate::processFinishedCall (call=0x7fd07c025470) at qdbusintegrator.cpp:1789
#7  0x00007fd0e3649e5a in complete_pending_call_and_unlock (connection=0x64d860, pending=0x7fd07c028930, message=<optimized out>) at dbus-connection.c:2304
#8  0x00007fd0e364cfd3 in dbus_connection_dispatch (connection=0x64d860) at dbus-connection.c:4552
#9  0x00007fd0eacb3379 in q_dbus_connection_dispatch (connection=<optimized out>) at qdbus_symbols_p.h:115
#10 QDBusConnectionPrivate::doDispatch (this=0x64cae0) at qdbusintegrator.cpp:1129
#11 0x00007fd0eacb7345 in QDBusConnectionPrivate::socketRead (this=0x64cae0, fd=<optimized out>) at qdbusintegrator.cpp:1149
#12 0x00007fd0ee2b8761 in QMetaObject::activate (sender=0xad8420, m=<optimized out>, local_signal_index=<optimized out>, argv=0x7fff84791ae0) at kernel/qobject.cpp:3556
#13 0x00007fd0ee305c6e in QSocketNotifier::activated (this=<optimized out>, _t1=5) at .moc/release-shared/moc_qsocketnotifier.cpp:103
#14 0x00007fd0ee2c1aeb in QSocketNotifier::event (this=0xad8420, e=0x7fff84792150) at kernel/qsocketnotifier.cpp:317
#15 0x00007fd0ed629e84 in notify_helper (e=0x7fff84792150, receiver=0xad8420, this=0x662510) at kernel/qapplication.cpp:4550
#16 QApplicationPrivate::notify_helper (this=0x662510, receiver=0xad8420, e=0x7fff84792150) at kernel/qapplication.cpp:4522
#17 0x00007fd0ed62ed03 in QApplication::notify (this=0x7fff847924b0, receiver=0xad8420, e=0x7fff84792150) at kernel/qapplication.cpp:4411
#18 0x00007fd0ef353656 in KApplication::notify (this=0x7fff847924b0, receiver=0xad8420, event=0x7fff84792150) at /usr/src/debug/kdelibs-4.8.1/kdeui/kernel/kapplication.cpp:311
#19 0x00007fd0ee2a489c in QCoreApplication::notifyInternal (this=0x7fff847924b0, receiver=0xad8420, event=0x7fff84792150) at kernel/qcoreapplication.cpp:876
#20 0x00007fd0ee2d2f47 in sendEvent (event=0x7fff84792150, receiver=<optimized out>) at kernel/qcoreapplication.h:231
#21 socketNotifierSourceDispatch (source=0x664d90) at kernel/qeventdispatcher_glib.cpp:110
#22 0x00007fd0e4e5658d in g_main_dispatch (context=0x664c20) at gmain.c:2425
#23 g_main_context_dispatch (context=0x664c20) at gmain.c:2995
#24 0x00007fd0e4e56d88 in g_main_context_iterate (context=0x664c20, block=<optimized out>, dispatch=1, self=<optimized out>) at gmain.c:3073
#25 0x00007fd0e4e56f59 in g_main_context_iteration (context=0x664c20, may_block=1) at gmain.c:3136
#26 0x00007fd0ee2d38ef in QEventDispatcherGlib::processEvents (this=0x60b000, flags=<optimized out>) at kernel/qeventdispatcher_glib.cpp:424
#27 0x00007fd0ed6cd2de in QGuiEventDispatcherGlib::processEvents (this=<optimized out>, flags=<optimized out>) at kernel/qguieventdispatcher_glib.cpp:204
#28 0x00007fd0ee2a3682 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:149
#29 0x00007fd0ee2a38d7 in QEventLoop::exec (this=0x7fff84792390, flags=...) at kernel/qeventloop.cpp:204
#30 0x00007fd0ee2a8435 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1148
#31 0x0000000000402df3 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/kdepim-4.8.1/kmail/main.cpp:145