The last two versions of kmail I've used always crash on startup. Current version is 4.8.4 "release 511". 
The messages I get are as follows:
***
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
kmail2(17002)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
kmail2(17002)/kdepimlibs (mailtransport): Could not access Outbox. 
QObject: Cannot create children for a parent that is in a different thread.
(Parent is Nepomuk::ResourceManager(0x90d580), parent's thread is QThread(0x60b7b0), current thread is QThread(0xb8b640)
*** KMail got signal 11 (Exiting)
*** Dead letters dumped.
***
Application: kmail (4.8.4)
KDE Platform Version: 4.8.4 (4.8.4) "release 511"
Qt Version: 4.8.2
Operating System: Linux 3.3.4 x86_64
Distribution: "openSUSE 12.1 (x86_64)"

-- Information about the crash:
<In detail, tell us what you were doing  when the application crashed.>

The crash can be reproduced every time.

-- Backtrace:
Application: KMail (kmail), signal: Segmentation fault
[Current thread is 1 (Thread 0x7fa6cfd5b780 (LWP 17002))]

Thread 5 (Thread 0x7fa6b14ad700 (LWP 17011)):
#0  0x00007fa6caebbe6c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007fa6c155ac12 in WTF::TCMalloc_PageHeap::scavengerThread (this=0x7fa6c1ef91a0) at ../../../Source/JavaScriptCore/wtf/FastMalloc.cpp:2495
#2  0x00007fa6c155ac49 in WTF::TCMalloc_PageHeap::runScavengerThread (context=<optimized out>) at ../../../Source/JavaScriptCore/wtf/FastMalloc.cpp:1618
#3  0x00007fa6caeb7f05 in start_thread () from /lib64/libpthread.so.0
#4  0x00007fa6cd0ab10d in clone () from /lib64/libc.so.6

Thread 4 (Thread 0x7fa6b0b94700 (LWP 17012)):
#0  0x00007fa6caebb11f in __pthread_mutex_unlock_usercnt () from /lib64/libpthread.so.0
#1  0x00007fa6c52b17a1 in g_mutex_unlock () from /opt/gnome/lib64/libglib-2.0.so.0
#2  0x00007fa6c52767ea in g_main_context_iterate.isra.23 () from /opt/gnome/lib64/libglib-2.0.so.0
#3  0x00007fa6c5276924 in g_main_context_iteration () from /opt/gnome/lib64/libglib-2.0.so.0
#4  0x00007fa6ce709e36 in QEventDispatcherGlib::processEvents (this=0x7fa6ac0008c0, flags=<optimized out>) at kernel/qeventdispatcher_glib.cpp:426
#5  0x00007fa6ce6d9842 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:149
#6  0x00007fa6ce6d9a97 in QEventLoop::exec (this=0x7fa6b0b93e00, flags=...) at kernel/qeventloop.cpp:204
#7  0x00007fa6ce5d95f7 in QThread::exec (this=<optimized out>) at thread/qthread.cpp:501
#8  0x00007fa6ce5dc61b in QThreadPrivate::start (arg=0xc1a580) at thread/qthread_unix.cpp:307
#9  0x00007fa6caeb7f05 in start_thread () from /lib64/libpthread.so.0
#10 0x00007fa6cd0ab10d in clone () from /lib64/libc.so.6

Thread 3 (Thread 0x7fa66af1e700 (LWP 17018)):
#0  0x00007fa6caebc1eb in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007fa6ce5dca8f in wait (time=30000, this=0xc29b00) at thread/qwaitcondition_unix.cpp:84
#2  QWaitCondition::wait (this=<optimized out>, mutex=0xc29a78, time=30000) at thread/qwaitcondition_unix.cpp:158
#3  0x00007fa6ce5cfe9f in QThreadPoolThread::run (this=0xb8b640) at concurrent/qthreadpool.cpp:141
#4  0x00007fa6ce5dc61b in QThreadPrivate::start (arg=0xb8b640) at thread/qthread_unix.cpp:307
#5  0x00007fa6caeb7f05 in start_thread () from /lib64/libpthread.so.0
#6  0x00007fa6cd0ab10d in clone () from /lib64/libc.so.6

Thread 2 (Thread 0x7fa66a18d700 (LWP 17024)):
#0  0x00007fa6caebc1eb in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007fa6ce5dca8f in wait (time=30000, this=0x1072cf0) at thread/qwaitcondition_unix.cpp:84
#2  QWaitCondition::wait (this=<optimized out>, mutex=0x10768e8, time=30000) at thread/qwaitcondition_unix.cpp:158
#3  0x00007fa6ce5cfe9f in QThreadPoolThread::run (this=0x1077510) at concurrent/qthreadpool.cpp:141
#4  0x00007fa6ce5dc61b in QThreadPrivate::start (arg=0x1077510) at thread/qthread_unix.cpp:307
#5  0x00007fa6caeb7f05 in start_thread () from /lib64/libpthread.so.0
#6  0x00007fa6cd0ab10d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7fa6cfd5b780 (LWP 17002)):
[KCrash Handler]
#6  0x00007fa6cf1fcfff in ?? () from /usr/lib64/libkdecore.so.5
#7  0x00007fa6ce5da5ed in QThreadStorageData::set (this=0x1125ec0, p=0xa33090) at thread/qthreadstorage.cpp:165
#8  0x00007fa6cf1fbd77 in ?? () from /usr/lib64/libkdecore.so.5
#9  0x00007fa6cf1fb2e2 in KServiceTypeTrader::applyConstraints(QList<KSharedPtr<KService> >&, QString const&) () from /usr/lib64/libkdecore.so.5
#10 0x00007fa6cf1fb4ce in KServiceTypeTrader::defaultOffers(QString const&, QString const&) const () from /usr/lib64/libkdecore.so.5
#11 0x00007fa6cf1fbc21 in KServiceTypeTrader::query(QString const&, QString const&) const () from /usr/lib64/libkdecore.so.5
#12 0x00007fa6befee9de in ?? () from /usr/lib64/libkemoticons.so.4
#13 0x00007fa6befeeb3a in KEmoticons::KEmoticons() () from /usr/lib64/libkemoticons.so.4
#14 0x00007fa6c6929502 in KPIMUtils::LinkLocator::convertToHtml(QString const&, int, int, int) () from /usr/lib64/libkpimutils.so.4
#15 0x00007fa6c6448d04 in MessageViewer::strToHtml (str=<optimized out>, flags=<optimized out>) at /usr/src/debug/kdepim-4.8.4/messageviewer/headerstyle.cpp:84
#16 0x00007fa6c6448fc0 in MessageViewer::subjectString (message=0x10cf4e0, flags=3) at /usr/src/debug/kdepim-4.8.4/messageviewer/headerstyle.cpp:111
#17 0x00007fa6c645129c in MessageViewer::FancyHeaderStyle::format (this=0xbd07c0, message=0x10cf4e0) at /usr/src/debug/kdepim-4.8.4/messageviewer/headerstyle.cpp:612
#18 0x00007fa6c64a09df in MessageViewer::ViewerPrivate::writeMsgHeader (this=0xbbfe10, aMsg=0x10cf4e0, vCardNode=<optimized out>, topLevel=true) at /usr/src/debug/kdepim-4.8.4/messageviewer/viewer_p.cpp:968
#19 0x00007fa6c64ad8bd in MessageViewer::ViewerPrivate::parseContent (this=0xbbfe10, content=0x10cf4e0) at /usr/src/debug/kdepim-4.8.4/messageviewer/viewer_p.cpp:907
#20 0x00007fa6c64aed1e in MessageViewer::ViewerPrivate::displayMessage (this=0xbbfe10) at /usr/src/debug/kdepim-4.8.4/messageviewer/viewer_p.cpp:790
#21 0x00007fa6c64af532 in MessageViewer::ViewerPrivate::updateReaderWin (this=0xbbfe10) at /usr/src/debug/kdepim-4.8.4/messageviewer/viewer_p.cpp:2084
#22 0x00007fa6c64b6c5a in MessageViewer::ViewerPrivate::qt_static_metacall (_o=0xbbfe10, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at /usr/src/debug/kdepim-4.8.4/build/messageviewer/viewer_p.moc:208
#23 0x00007fa6ce6eec41 in QMetaObject::activate (sender=0xbbfe98, m=<optimized out>, local_signal_index=<optimized out>, argv=0x0) at kernel/qobject.cpp:3556
#24 0x00007fa6ce6f3b39 in QObject::event (this=0xbbfe98, e=<optimized out>) at kernel/qobject.cpp:1166
#25 0x00007fa6cda5dfe4 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib64/libQtGui.so.4
#26 0x00007fa6cda62db3 in QApplication::notify(QObject*, QEvent*) () from /usr/lib64/libQtGui.so.4
#27 0x00007fa6cf791746 in KApplication::notify(QObject*, QEvent*) () from /usr/lib64/libkdeui.so.5
#28 0x00007fa6ce6dad0c in QCoreApplication::notifyInternal (this=0x7fff1d3edef0, receiver=0xbbfe98, event=0x7fff1d3edaf0) at kernel/qcoreapplication.cpp:915
#29 0x00007fa6ce70bbe8 in sendEvent (event=0x7fff1d3edaf0, receiver=<optimized out>) at kernel/qcoreapplication.h:231
#30 QTimerInfoList::activateTimers (this=0x66a060) at kernel/qeventdispatcher_unix.cpp:626
#31 0x00007fa6ce70959d in timerSourceDispatch (source=<optimized out>) at kernel/qeventdispatcher_glib.cpp:186
#32 timerSourceDispatch (source=<optimized out>) at kernel/qeventdispatcher_glib.cpp:180
#33 0x00007fa6c527649a in g_main_context_dispatch () from /opt/gnome/lib64/libglib-2.0.so.0
#34 0x00007fa6c5276860 in g_main_context_iterate.isra.23 () from /opt/gnome/lib64/libglib-2.0.so.0
#35 0x00007fa6c5276924 in g_main_context_iteration () from /opt/gnome/lib64/libglib-2.0.so.0
#36 0x00007fa6ce709dcf in QEventDispatcherGlib::processEvents (this=0x60b000, flags=<optimized out>) at kernel/qeventdispatcher_glib.cpp:424
#37 0x00007fa6cdb0169e in ?? () from /usr/lib64/libQtGui.so.4
#38 0x00007fa6ce6d9842 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:149
#39 0x00007fa6ce6d9a97 in QEventLoop::exec (this=0x7fff1d3eddd0, flags=...) at kernel/qeventloop.cpp:204
#40 0x00007fa6ce6de8b5 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1187
#41 0x0000000000402df3 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/kdepim-4.8.4/kmail/main.cpp:145


Reproducible: Always

Steps to Reproduce:
1. Open kmail, either on console or from menu
2. Application crashes after a brief display of the UI
3.
Actual Results:  
Instant crash

Expected Results:  
Display mail accounts/folders

Incidentally, I also use Kmail (KDE3) on the same mail folders, which is a lot more stable, but I suspect that the two kmails clash on how they mark the messages (deleted, read, ..)