Application: kmail (1.13.1)
KDE Platform Version: 4.4.1 (KDE 4.4.1)
Qt Version: 4.6.2
Operating System: Linux 2.6.32-16-generic i686
Distribution: Ubuntu lucid (development branch)

-- Information about the crash:
I've setup Thunderbird to use local folders and used the directory of KMail.
However, I was using Maildir format with KMail and Thunderbird cannot handle this. 

Instead, it appears to have created a mbox setup and a set of *.sbd folders.
The .sbd folders made KMail popup an error for every one of them (twice during startup) and a lot more afterwards (I had to hold Escape to dismiss them all).
Then, KMail crashed, before I had access anything (IIRC).

 -- Backtrace:
Application: KMail (kmail), signal: Segmentation fault
[Current thread is 1 (Thread 0xb78b7ac0 (LWP 24806))]

Thread 2 (Thread 0xb3616b70 (LWP 25560)):
#0  0x00848422 in __kernel_vsyscall ()
#1  0x087d9af6 in *__GI___poll (fds=0x8869ff4, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:87
#2  0x054a432b in g_poll () from /lib/libglib-2.0.so.0
#3  0x05496e5c in ?? () from /lib/libglib-2.0.so.0
#4  0x05497268 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#5  0x00a1559f in QEventDispatcherGlib::processEvents (this=0xb006970, flags=...) at kernel/qeventdispatcher_glib.cpp:414
#6  0x009e7ff9 in QEventLoop::processEvents (this=0xb3616230, flags=) at kernel/qeventloop.cpp:149
#7  0x009e844a in QEventLoop::exec (this=0xb3616230, flags=...) at kernel/qeventloop.cpp:201
#8  0x008e45a8 in QThread::exec (this=0xb0133d0) at thread/qthread.cpp:487
#9  0x009c7bcb in QInotifyFileSystemWatcherEngine::run (this=0xb0133d0) at io/qfilesystemwatcher_inotify.cpp:248
#10 0x008e732e in QThreadPrivate::start (arg=0xb0133d0) at thread/qthread_unix.cpp:248
#11 0x0084e96e in start_thread (arg=0xb3616b70) at pthread_create.c:300
#12 0x087e792e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130

Thread 1 (Thread 0xb78b7ac0 (LWP 24806)):
[KCrash Handler]
#6  0x01192723 in QPointer<KMFolder>::operator KMFolder* (this=0xb0a5858) at /usr/include/qt4/QtCore/qpointer.h:78
#7  KMSystemTray::updateNewMessages (this=0xb0a5858) at ../../kmail/kmsystemtray.cpp:387
#8  0x01195223 in KMSystemTray::qt_metacall (this=0xb0a5858, _c=QMetaObject::InvokeMetaMethod, _id=19, _a=0xbfbd0a20) at ./kmsystemtray.moc:84
#9  0x009eec3a in QMetaObject::metacall (object=0xb0a5858, cl=QMetaObject::ReadProperty, idx=19, argv=0xbfbd0a20) at kernel/qmetaobject.cpp:237
#10 0x009fd375 in QMetaObject::activate (sender=0x964cba8, m=0x171af84, local_signal_index=0, argv=0xbfbd0a20) at kernel/qobject.cpp:3293
#11 0x0118ceeb in KMail::AccountManager::checkedMail (this=0x964cba8, _t1=false, _t2=true, _t3=...) at ./accountmanager.moc:115
#12 0x0118ec98 in KMail::AccountManager::processNextCheck (this=0x964cba8, _newMail=false) at ../../kmail/accountmanager.cpp:184
#13 0x0118f220 in KMail::AccountManager::singleCheckMail (this=0x964cba8, account=0x0, interactive=true) at ../../kmail/accountmanager.cpp:157
#14 0x0118f510 in KMail::AccountManager::checkMail (this=0x964cba8, _interactive=true) at ../../kmail/accountmanager.cpp:352
#15 0x01350688 in KMMainWidget::slotCheckMail (this=0xab0eb20) at ../../kmail/kmmainwidget.cpp:1037
#16 0x013750d8 in KMMainWidget::qt_metacall (this=0xab0eb20, _c=QMetaObject::InvokeMetaMethod, _id=9, _a=0xbfbd0d1c) at ./kmmainwidget.moc:363
#17 0x009eec3a in QMetaObject::metacall (object=0xab0eb20, cl=QMetaObject::ReadProperty, idx=36, argv=0xbfbd0d1c) at kernel/qmetaobject.cpp:237
#18 0x009fd375 in QMetaObject::activate (sender=0xb07bd20, m=0xafc188, local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3293
#19 0x00a04f87 in QSingleShotTimer::timeout (this=0xb07bd20) at .moc/release-shared/qtimer.moc:82
#20 0x00a0509c in QSingleShotTimer::timerEvent (this=0xb07bd20) at kernel/qtimer.cpp:308
#21 0x009fa1f4 in QObject::event (this=0xb07bd20, e=0x0) at kernel/qobject.cpp:1212
#22 0x061594dc in QApplicationPrivate::notify_helper (this=0x93d6808, receiver=0xb07bd20, e=0xbfbd1250) at kernel/qapplication.cpp:4300
#23 0x0616005e in QApplication::notify (this=0xbfbd1660, receiver=0xb07bd20, e=0xbfbd1250) at kernel/qapplication.cpp:3704
#24 0x00df2efa in KApplication::notify (this=0xbfbd1660, receiver=0xb07bd20, event=0xbfbd1250) at ../../kdeui/kernel/kapplication.cpp:302
#25 0x009e99db in QCoreApplication::notifyInternal (this=0xbfbd1660, receiver=0xb07bd20, event=0xbfbd1250) at kernel/qcoreapplication.cpp:704
#26 0x00a18cf6 in QCoreApplication::sendEvent (this=0x93dea84) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:215
#27 QTimerInfoList::activateTimers (this=0x93dea84) at kernel/qeventdispatcher_unix.cpp:603
#28 0x00a15874 in timerSourceDispatch (source=0x93dea50) at kernel/qeventdispatcher_glib.cpp:184
#29 0x05493395 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
#30 0x05497088 in ?? () from /lib/libglib-2.0.so.0
#31 0x05497268 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#32 0x00a15565 in QEventDispatcherGlib::processEvents (this=0x93a4278, flags=...) at kernel/qeventdispatcher_glib.cpp:412
#33 0x06218f65 in QGuiEventDispatcherGlib::processEvents (this=0x93a4278, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
#34 0x009e7ff9 in QEventLoop::processEvents (this=0xbfbd1514, flags=) at kernel/qeventloop.cpp:149
#35 0x009e844a in QEventLoop::exec (this=0xbfbd1514, flags=...) at kernel/qeventloop.cpp:201
#36 0x009ec63f in QCoreApplication::exec () at kernel/qcoreapplication.cpp:981
#37 0x06159577 in QApplication::exec () at kernel/qapplication.cpp:3579
#38 0x0804ab0e in main (argc=3, argv=0xbfbd1824) at ../../kmail/main.cpp:156

Reported using DrKonqi