This seems to be related to
Bug 289933
Deactivating Prefer HTML to plain text' and 'Allow messages to load external references from the Internet' also fixes Bug 289933.

could you send me your email which crashs kmail please in private message.

done

I can't reproduce
could you send me backtrace please ?
thanks

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff26a6ff4 in QNetworkAccessHttpBackend::replyDownloadMetaData (this=0x180cf30, hm=..., sc=304, rp=..., pu=false, db=..., contentLength=-1) at access/qnetworkaccesshttpbackend.cpp:827
827                 QNetworkCacheMetaData oldMetaData = nc->metaData(url());
(gdb) where
#0  0x00007ffff26a6ff4 in QNetworkAccessHttpBackend::replyDownloadMetaData (this=0x180cf30, hm=..., sc=304, rp=..., pu=false, db=..., contentLength=-1) at access/qnetworkaccesshttpbackend.cpp:827
#1  0x00007ffff2739e2d in QNetworkAccessHttpBackend::qt_static_metacall (_o=0x180cf30, _c=QMetaObject::InvokeMetaMethod, _id=6, _a=0x7fff80007c80) at .moc/debug-shared/moc_qnetworkaccesshttpbackend_p.cpp:91
#2  0x00007ffff1d82b57 in QMetaCallEvent::placeMetaCall (this=0x7fff800076b0, object=0x180cf30) at kernel/qobject.cpp:525
#3  0x00007ffff1d83ddd in QObject::event (this=0x180cf30, e=0x7fff800076b0) at kernel/qobject.cpp:1195
#4  0x00007ffff2c0bebe in QApplicationPrivate::notify_helper (this=0x65c550, receiver=0x180cf30, e=0x7fff800076b0) at kernel/qapplication.cpp:4550
#5  0x00007ffff2c094fa in QApplication::notify (this=0x7fffffffd580, receiver=0x180cf30, e=0x7fff800076b0) at kernel/qapplication.cpp:3932
#6  0x00007ffff79a2636 in KApplication::notify (this=0x7fffffffd580, receiver=0x180cf30, event=0x7fff800076b0) at /source/kde/kde4/kdelibs/kdeui/kernel/kapplication.cpp:311
#7  0x00007ffff1d6af2e in QCoreApplication::notifyInternal (this=0x7fffffffd580, receiver=0x180cf30, event=0x7fff800076b0) at kernel/qcoreapplication.cpp:876
#8  0x00007ffff4ce95ab in QCoreApplication::sendEvent (receiver=0x180cf30, event=0x7fff800076b0) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#9  0x00007ffff1d6bf42 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x60b740) at kernel/qcoreapplication.cpp:1500
#10 0x00007ffff1d6bb63 in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1393
#11 0x00007ffff2cd6dfd in QCoreApplication::sendPostedEvents () at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:236
#12 0x00007ffff1da67c4 in postEventSourceDispatch (s=0x661d50) at kernel/qeventdispatcher_glib.cpp:279
#13 0x00007fffe699e58d in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#14 0x00007fffe699ed88 in ?? () from /usr/lib64/libglib-2.0.so.0
#15 0x00007fffe699ef59 in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#16 0x00007ffff1da709b in QEventDispatcherGlib::processEvents (this=0x60b000, flags=...) at kernel/qeventdispatcher_glib.cpp:424
#17 0x00007ffff2ce2fe8 in QGuiEventDispatcherGlib::processEvents (this=0x60b000, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
#18 0x00007ffff1d687f8 in QEventLoop::processEvents (this=0x7fffffffd450, flags=...) at kernel/qeventloop.cpp:149
#19 0x00007ffff1d68982 in QEventLoop::exec (this=0x7fffffffd450, flags=...) at kernel/qeventloop.cpp:204
#20 0x00007ffff1d6b5ae in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1148
#21 0x00007ffff2c090e4 in QApplication::exec () at kernel/qapplication.cpp:3811
#22 0x0000000000402fa3 in main (argc=<optimized out>, argv=<optimized out>) at /source/kde/kde4/kdepim/kmail/main.cpp:145
(gdb) 


Ok found this crash.
Crash in qt

dup of 286307 then

*** Bug 289933 has been marked as a duplicate of this bug. ***

*** Bug 291913 has been marked as a duplicate of this bug. ***

Created attachment 68183 [details]
New crash information added by DrKonqi

kmail (4.7.4) on KDE Platform 4.7.4 (4.7.4) using Qt 4.8.0

- What I was doing when the application crashed:
Looking at certain HTML emails causes KMail to crash.

- Custom settings of the application:
I have "prefer HTML to plain text" and "allow external references from the Internet" enabled.

-- Backtrace (Reduced):
#8  0xb6a457d9 in QNetworkAccessHttpBackend::replyDownloadMetaData (this=0xb352b58, hm=..., sc=-1076459432, rp=..., pu=false, db=..., contentLength=-1230115716) at access/qnetworkaccesshttpbackend.cpp:827
#9  0xb6ad7dad in QNetworkAccessHttpBackend::qt_static_metacall (_o=0xb352b58, _c=QMetaObject::InvokeMetaMethod, _id=6, _a=0xaa805a68) at .moc/release-shared/moc_qnetworkaccesshttpbackend_p.cpp:91
[...]
#11 0xb67dd42b in QObject::event (this=0xb352b58, e=0xaa802cc8) at kernel/qobject.cpp:1195
#12 0xb6c5db74 in notify_helper (e=0xaa802cc8, receiver=0xb352b58, this=0x98d1428) at kernel/qapplication.cpp:4550
#13 QApplicationPrivate::notify_helper (this=0x98d1428, receiver=0xb352b58, e=0xaa802cc8) at kernel/qapplication.cpp:4522

*** Bug 292546 has been marked as a duplicate of this bug. ***

I have this problem as well. It *only* seems to happen to me, if KMail has been sitting for awhile. I can restart KMail immediately after a crash, and load HTML messages (not even outside content, just activating HTML) and it will not crash. However, the minute I have had KMail sitting for awhile (at least a couple hours) and try to activate an HTML message, it crashes.

*** This bug has been marked as a duplicate of bug 286307 ***

*** Bug 292991 has been marked as a duplicate of this bug. ***