sorry, here's the backtrace:

Application: Kontact (kontact), signal SIGABRT
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
[Thread debugging using libthread_db enabled]
[New Thread -1256782144 (LWP 11632)]
[KCrash handler]
#6  0xffffe410 in __kernel_vsyscall ()
#7  0xb557b875 in raise () from /lib/tls/i686/cmov/libc.so.6
#8  0xb557d201 in abort () from /lib/tls/i686/cmov/libc.so.6
#9  0xb7d5f9d5 in qt_message_output (msgType=QtFatalMsg, 
    buf=0xbfd16bb0 "ASSERT: \"false\" in file /home/jeremy/devel/kde4/src/kdepimlibs/mailtransport/transportmanager.cpp, line 228")
    at global/qglobal.cpp:2047
#10 0xb7d5fa59 in qFatal (msg=0xb7ea37f4 "ASSERT: \"%s\" in file %s, line %d")
    at global/qglobal.cpp:2249
#11 0xb7d5fec5 in qt_assert (assertion=0xb2c03e50 "false", 
    file=0xb2c03d10 "/home/jeremy/devel/kde4/src/kdepimlibs/mailtransport/transportmanager.cpp", line=228) at global/qglobal.cpp:1821
#12 0xb2bef33f in MailTransport::TransportManager::createTransportJob (
    this=0x83d8a90, transport=@0xbfd18cf0)
    at /home/jeremy/devel/kde4/src/kdepimlibs/mailtransport/transportmanager.cpp:228
#13 0xb2e39adc in KMSender::doSendMsg (this=0x8453178)
    at /home/jeremy/devel/kde4/src/kdepim/kmail/kmsender.cpp:535
#14 0xb2e3af2e in KMSender::doSendQueued (this=0x8453178, 
    customTransport=@0xbfd18eac)
    at /home/jeremy/devel/kde4/src/kdepim/kmail/kmsender.cpp:251
#15 0xb2e3b6cd in KMail::MessageSender::sendQueued (this=0x8453180, 
    transport=@0xbfd18eac)
    at /home/jeremy/devel/kde4/src/kdepim/kmail/messagesender.h:77
#16 0xb2fb9da5 in KMMainWidget::slotSendQueued (this=0x8414dd8)
    at /home/jeremy/devel/kde4/src/kdepim/kmail/kmmainwidget.cpp:2048
#17 0xb2fda0af in KMMainWidget::qt_metacall (this=0x8414dd8, 
    _c=QMetaObject::InvokeMetaMethod, _id=97, _a=0xbfd18fec)
    at /home/jeremy/devel/kde4/build/kdepim/kmail/kmmainwidget.moc:413
#18 0xb7e63a6b in QMetaObject::activate (sender=0x850a3b8, 
    from_signal_index=5, to_signal_index=6, argv=0xbfd18fec)
    at kernel/qobject.cpp:2992
#19 0xb7e63c6e in QMetaObject::activate (sender=0x850a3b8, m=0xb7406e58, 
    from_local_signal_index=1, to_local_signal_index=2, argv=0xbfd18fec)
    at kernel/qobject.cpp:3085
#20 0xb6d18bd2 in QAction::triggered (this=0x850a3b8, _t1=false)
    at .moc/debug-shared/moc_qaction.cpp:216
#21 0xb6d1aadc in QAction::activate (this=0x850a3b8, event=QAction::Trigger)
    at kernel/qaction.cpp:1129
#22 0xb7138c4c in QMenuPrivate::activateAction (this=0x867b938, 
    action=0x850a3b8, action_e=QAction::Trigger, self=true)
    at widgets/qmenu.cpp:1005
#23 0xb713b5f9 in QMenu::mouseReleaseEvent (this=0x83d96e0, e=0xbfd19858)
    at widgets/qmenu.cpp:2144
#24 0xb7a41a34 in KMenu::mouseReleaseEvent (this=0x83d96e0, e=0xbfd19858)
    at /home/jeremy/devel/kde4/src/kdelibs/kdeui/widgets/kmenu.cpp:423
#25 0xb6d7e21f in QWidget::event (this=0x83d96e0, event=0xbfd19858)
    at kernel/qwidget.cpp:6808
#26 0xb7136890 in QMenu::event (this=0x83d96e0, e=0xbfd19858)
    at widgets/qmenu.cpp:2240
#27 0xb6d208d7 in QApplicationPrivate::notify_helper (this=0x8067288, 
    receiver=0x83d96e0, e=0xbfd19858) at kernel/qapplication.cpp:3757
#28 0xb6d213de in QApplication::notify (this=0xbfd1b2dc, receiver=0x83d96e0, 
    e=0xbfd19858) at kernel/qapplication.cpp:3486
#29 0xb797e22f in KApplication::notify (this=0xbfd1b2dc, receiver=0x83d96e0, 
    event=0xbfd19858)
    at /home/jeremy/devel/kde4/src/kdelibs/kdeui/kernel/kapplication.cpp:311
#30 0xb7e4fdc0 in QCoreApplication::notifyInternal (this=0xbfd1b2dc, 
    receiver=0x83d96e0, event=0xbfd19858) at kernel/qcoreapplication.cpp:582
#31 0xb6d2c813 in QCoreApplication::sendSpontaneousEvent (receiver=0x83d96e0, 
    event=0xbfd19858)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
#32 0xb6d27722 in QApplicationPrivate::sendMouseEvent (receiver=0x83d96e0, 
    event=0xbfd19858, alienWidget=0x0, nativeWidget=0x83d96e0, 
    buttonDown=0xb74249e0, lastMouseReceiver=@0xb74249e4)
    at kernel/qapplication.cpp:2751
#33 0xb6d9a66d in QETWidget::translateMouseEvent (this=0x83d96e0, 
    event=0xbfd19e3c) at kernel/qapplication_x11.cpp:4087
#34 0xb6d9800d in QApplication::x11ProcessEvent (this=0xbfd1b2dc, 
    event=0xbfd19e3c) at kernel/qapplication_x11.cpp:3153
#35 0xb6dc772d in x11EventSourceDispatch (s=0x8068740, callback=0, 
    user_data=0x0) at kernel/qguieventdispatcher_glib.cpp:148
#36 0xb54b511c in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#37 0xb54b855f in ?? () from /usr/lib/libglib-2.0.so.0
#38 0x08066e58 in ?? ()
#39 0x00000000 in ?? ()
#0  0xffffe410 in __kernel_vsyscall ()

The reason for the assert is that the transport manager can't find the transport that was passed to it (transport = sending account).

This doesn't happen for me, so please answer the following questions:

What is the name of the sending account you are using?
How did you send the message? Directly from the composer or by queing it first?
Are you using completely new configs or did you re-use your old configs from KDE3?
Is this problem reproduceable with a clean config? If so, please add the minimal steps to reproduce here. If not, your config file would probably help, please attach .kde[4]/share/config/mailtransports (strip the passwords first!).

Thanks.

weird I swear I was getting this on both computers...
I just tried here making sure I had a clean config and it works fine
I'll try on the work pc tommorow, but I bet I had a config from kde3 there by
mistake also

*** Bug has been marked as fixed ***.

*** Bug 165185 has been marked as a duplicate of this bug. ***