In fact problem is appearing because I was added custom header 'Content-type: text/html; charset=utf-8' to 'trick' KMail to send HTML formatted e-mail messages.

After removing customer header - problem is not appearing.

(In reply to comment #0)
> Application: kontact (4.12.3)
> KDE Platform Version: 4.12.90
> Qt Version: 4.8.6
> Operating System: Linux 3.13.0-18-generic i686
> Distribution: Ubuntu Trusty Tahr (development branch)
> 
> -- Information about the crash:
> - What I was doing when the application crashed:
> I was trying to format text in Composer windows when suddenly KMail got
> crashed.
> 
> -- Backtrace:
> Application: Kontact (kontact), signal: Segmentation fault
> Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
> [Current thread is 1 (Thread 0xb16ae740 (LWP 2363))]
> 
> Thread 6 (Thread 0xaea92b40 (LWP 2364)):
> #0  0xb7798424 in __kernel_vsyscall ()
> #1  0xb323fd1b in pthread_cond_wait@@GLIBC_2.3.2 () at
> ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/pthread_cond_wait.S:187
> #2  0xb6125bbc in __pthread_cond_wait (cond=0xb58acd90, mutex=0xb58acd78) at
> forward.c:149
> #3  0xb503280c in ?? () from /usr/lib/i386-linux-gnu/libQtWebKit.so.4
> #4  0xb503285f in ?? () from /usr/lib/i386-linux-gnu/libQtWebKit.so.4
> #5  0xb323bf70 in start_thread (arg=0xaea92b40) at pthread_create.c:312
> #6  0xb6118a6e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129
> 
> Thread 5 (Thread 0xae171b40 (LWP 2365)):
> #0  0xb7798424 in __kernel_vsyscall ()
> #1  0xb323fd1b in pthread_cond_wait@@GLIBC_2.3.2 () at
> ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/pthread_cond_wait.S:187
> #2  0xb6125bbc in __pthread_cond_wait (cond=0xae1df7dc, mutex=0xae1df7c4) at
> forward.c:149
> #3  0xb50618d3 in WTF::ThreadCondition::wait(WTF::Mutex&) () from
> /usr/lib/i386-linux-gnu/libQtWebKit.so.4
> #4  0xb4d3c6f4 in ?? () from /usr/lib/i386-linux-gnu/libQtWebKit.so.4
> #5  0xb5048616 in ?? () from /usr/lib/i386-linux-gnu/libQtWebKit.so.4
> #6  0xb506124b in ?? () from /usr/lib/i386-linux-gnu/libQtWebKit.so.4
> #7  0xb323bf70 in start_thread (arg=0xae171b40) at pthread_create.c:312
> #8  0xb6118a6e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129
> 
> Thread 4 (Thread 0xa9afeb40 (LWP 2379)):
> #0  0xb7798424 in __kernel_vsyscall ()
> #1  0xb6126662 in __GI___clock_gettime (clock_id=1, tp=0xa9afe028) at
> ../sysdeps/unix/clock_gettime.c:115
> #2  0xb63878cc in do_gettime (frac=0xa9afe020, sec=0xa9afe018) at
> tools/qelapsedtimer_unix.cpp:127
> #3  qt_gettime () at tools/qelapsedtimer_unix.cpp:144
> #4  0xb64702a2 in updateCurrentTime (this=0xaa3044bc) at
> kernel/qeventdispatcher_unix.cpp:354
> #5  QTimerInfoList::timerWait (this=0xaa3044bc, tm=...) at
> kernel/qeventdispatcher_unix.cpp:460
> #6  0xb646e82b in timerSourcePrepareHelper (src=<optimized out>,
> timeout=0xa9afe11c) at kernel/qeventdispatcher_glib.cpp:143
> #7  0xb646e8bd in timerSourcePrepare (source=0xaa304488, timeout=0xa9afe11c)
> at kernel/qeventdispatcher_glib.cpp:176
> #8  0xb31559b3 in g_main_context_prepare () from
> /lib/i386-linux-gnu/libglib-2.0.so.0
> #9  0xb31562df in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
> #10 0xb3156528 in g_main_context_iteration () from
> /lib/i386-linux-gnu/libglib-2.0.so.0
> #11 0xb646e95f in QEventDispatcherGlib::processEvents (this=0xaa304388,
> flags=...) at kernel/qeventdispatcher_glib.cpp:436
> #12 0xb643d823 in QEventLoop::processEvents (this=this@entry=0xa9afe288,
> flags=...) at kernel/qeventloop.cpp:149
> #13 0xb643db49 in QEventLoop::exec (this=this@entry=0xa9afe288, flags=...)
> at kernel/qeventloop.cpp:204
> #14 0xb632a23d in QThread::exec (this=this@entry=0x9d97dd8) at
> thread/qthread.cpp:537
> #15 0xb632a38b in QThread::run (this=0x9d97dd8) at thread/qthread.cpp:604
> #16 0xb632cb6f in QThreadPrivate::start (arg=0x9d97dd8) at
> thread/qthread_unix.cpp:349
> #17 0xb323bf70 in start_thread (arg=0xa9afeb40) at pthread_create.c:312
> #18 0xb6118a6e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129
> 
> Thread 3 (Thread 0xa80d1b40 (LWP 2395)):
> #0  0xb7798424 in __kernel_vsyscall ()
> #1  0xb6109b2b in poll () at ../sysdeps/unix/syscall-template.S:81
> #2  0xb316520b in g_poll () from /lib/i386-linux-gnu/libglib-2.0.so.0
> #3  0xb31563e8 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
> #4  0xb3156528 in g_main_context_iteration () from
> /lib/i386-linux-gnu/libglib-2.0.so.0
> #5  0xb646e95f in QEventDispatcherGlib::processEvents (this=0xad800478,
> flags=...) at kernel/qeventdispatcher_glib.cpp:436
> #6  0xb643d823 in QEventLoop::processEvents (this=this@entry=0xa80d1258,
> flags=...) at kernel/qeventloop.cpp:149
> #7  0xb643db49 in QEventLoop::exec (this=this@entry=0xa80d1258, flags=...)
> at kernel/qeventloop.cpp:204
> #8  0xb632a23d in QThread::exec (this=this@entry=0xa5f1248) at
> thread/qthread.cpp:537
> #9  0xb641dc44 in QInotifyFileSystemWatcherEngine::run (this=0xa5f1248) at
> io/qfilesystemwatcher_inotify.cpp:265
> #10 0xb632cb6f in QThreadPrivate::start (arg=0xa5f1248) at
> thread/qthread_unix.cpp:349
> #11 0xb323bf70 in start_thread (arg=0xa80d1b40) at pthread_create.c:312
> #12 0xb6118a6e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129
> 
> Thread 2 (Thread 0xa7867b40 (LWP 2400)):
> #0  0xb7798424 in __kernel_vsyscall ()
> #1  0xb323fd1b in pthread_cond_wait@@GLIBC_2.3.2 () at
> ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/pthread_cond_wait.S:187
> #2  0xb6125bbc in __pthread_cond_wait (cond=0xac9c88d0, mutex=0xac9c88b8) at
> forward.c:149
> #3  0xac8c5c92 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
> #4  0xac8c5cdf in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
> #5  0xb323bf70 in start_thread (arg=0xa7867b40) at pthread_create.c:312
> #6  0xb6118a6e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129
> 
> Thread 1 (Thread 0xb16ae740 (LWP 2363)):
> [KCrash Handler]
> #7  QByteArray::indexOf (this=this@entry=0xcb5eac0, ch=ch@entry=47 '/',
> from=<optimized out>, from@entry=0) at tools/qbytearray.cpp:2314
> #8  0xb2aa85d6 in KMime::Headers::ContentType::isSubtype
> (this=this@entry=0xcb5eaa0, subtype=subtype@entry=0xb2abb736 "signed") at
> ../../kmime/kmime_headers.cpp:1795
> #9  0xb2a7ef62 in KMime::isSigned (message=0xbccd0d8) at
> ../../kmime/kmime_util.cpp:983
> #10 0xacdad904 in
> MailTransport::MessageQueueJob::Private::outboxRequestResult
> (this=0xc8c4a58, job=0xc246400) at
> ../../mailtransport/messagequeuejob.cpp:149
> #11 0xacdadda2 in MailTransport::MessageQueueJob::qt_static_metacall
> (_o=_o@entry=0xbdacd70, _c=_c@entry=QMetaObject::InvokeMetaMethod, _id=1,
> _a=0xbfe69a58) at ./moc_messagequeuejob.cpp:51
> #12 0xb64540f7 in QMetaObject::activate (sender=sender@entry=0xc246400,
> m=m@entry=0xb68594cc <KJob::staticMetaObject>,
> local_signal_index=local_signal_index@entry=3, argv=argv@entry=0xbfe69a58)
> at kernel/qobject.cpp:3539
> #13 0xb66b5215 in KJob::result (this=this@entry=0xc246400,
> _t1=_t1@entry=0xc246400) at ./kjob.moc:207
> #14 0xb66b5262 in KJob::emitResult (this=this@entry=0xc246400) at
> ../../kdecore/jobs/kjob.cpp:318
> #15 0xb5ba83b0 in Akonadi::SpecialCollectionsRequestJob::doStart
> (this=0xc246400) at ../../akonadi/specialcollectionsrequestjob.cpp:340
> #16 0xb5b50b12 in Akonadi::JobPrivate::startQueued (this=0xc51e238) at
> ../../akonadi/job.cpp:166
> #17 0xb5b9324e in Akonadi::SessionPrivate::startJob
> (this=this@entry=0x9cf6ed0, job=job@entry=0xc246400) at
> ../../akonadi/session.cpp:280
> #18 0xb5b93b0f in Akonadi::SessionPrivate::doStartNext (this=0x9cf6ed0) at
> ../../akonadi/session.cpp:269
> #19 0xb64508aa in QMetaCallEvent::placeMetaCall (this=0xc9e6290,
> object=0x9fa5e58) at kernel/qobject.cpp:524
> #20 0xb64586fb in QObject::event (this=0x9fa5e58, e=0xc9e6290) at
> kernel/qobject.cpp:1194
> #21 0xb699a7f4 in QApplicationPrivate::notify_helper (this=0x9bde060,
> receiver=0x9fa5e58, e=0xc9e6290) at kernel/qapplication.cpp:4567
> #22 0xb69a12d3 in QApplication::notify (this=0xbfe6a1f8,
> receiver=receiver@entry=0x9fa5e58, e=e@entry=0xc9e6290) at
> kernel/qapplication.cpp:4353
> #23 0xb74ea094 in KApplication::notify (this=0xbfe6a1f8, receiver=0x9fa5e58,
> event=0xc9e6290) at ../../kdeui/kernel/kapplication.cpp:311
> #24 0xb643ee4a in QCoreApplication::notifyInternal (this=0xbfe6a1f8,
> receiver=receiver@entry=0x9fa5e58, event=event@entry=0xc9e6290) at
> kernel/qcoreapplication.cpp:953
> #25 0xb644270d in sendEvent (event=0xc9e6290, receiver=0x9fa5e58) at
> ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
> #26 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0,
> event_type=event_type@entry=0, data=0x9bb2fa0) at
> kernel/qcoreapplication.cpp:1577
> #27 0xb6442c1c in QCoreApplication::sendPostedEvents
> (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at
> kernel/qcoreapplication.cpp:1470
> #28 0xb646f23e in sendPostedEvents () at
> ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:236
> #29 postEventSourceDispatch (s=0x9bdd0e0) at
> kernel/qeventdispatcher_glib.cpp:287
> #30 0xb31561e3 in g_main_context_dispatch () from
> /lib/i386-linux-gnu/libglib-2.0.so.0
> #31 0xb3156468 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
> #32 0xb3156528 in g_main_context_iteration () from
> /lib/i386-linux-gnu/libglib-2.0.so.0
> #33 0xb646e93b in QEventDispatcherGlib::processEvents
> (this=this@entry=0x9bdd560, flags=...) at
> kernel/qeventdispatcher_glib.cpp:434
> #34 0xb6a4f5de in QGuiEventDispatcherGlib::processEvents (this=0x9bdd560,
> flags=...) at kernel/qguieventdispatcher_glib.cpp:204
> #35 0xb643d823 in QEventLoop::processEvents (this=this@entry=0xbfe6a148,
> flags=...) at kernel/qeventloop.cpp:149
> #36 0xb643db49 in QEventLoop::exec (this=this@entry=0xbfe6a148, flags=...)
> at kernel/qeventloop.cpp:204
> #37 0xb64438fe in QCoreApplication::exec () at
> kernel/qcoreapplication.cpp:1225
> #38 0xb6998a24 in QApplication::exec () at kernel/qapplication.cpp:3828
> #39 0x0804ab4c in main (argc=1, argv=0xbfe6a2d4) at
> ../../../kontact/src/main.cpp:219
> 
> Reported using DrKonqi