Application: kmail (1.13.7)
KDE Platform Version: 4.8.4 (4.8.4)
Qt Version: 4.8.2
Operating System: Linux 3.2.0-3-amd64 x86_64
Distribution: Debian GNU/Linux testing (wheezy)

-- Information about the crash:
- What I was doing when the application crashed:

* Wrote a note, and customized it using almost all possible options (RTF, widget colors, on which desktop should it appear, etc)
* Right click on the title bar, select "Mail..."
* In Kmail window clicked send - crash

Although despite actions that were taken I believe it has nothing to do with them, but with custom message tags I created. I might be completely mistaken, but I'm deducting it by looking at classes and their methods' names.

For sake of completeness:
I removed ALL default message tags (like "Friends", "Family", etc.)
Created bunch of new (6 to be exact) and each is applied using least sophisticated filter/condition editor(?) options - up to two or three conditions per filter.

Results: After restarting Kmail, it regenerated index and all tags on e-mails were gone

-- Backtrace:
Application: KMail (kmail), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[KCrash Handler]
#6  KMMessageTagMgr::find (this=0x0, aLabel=...) at ../../kmail/kmmessagetag.cpp:181
#7  0x00007f1abdaef330 in KMMessageTagList::compareTags (this=this@entry=0x13e4920, lhs=..., rhs=...) at ../../kmail/kmmessagetag.cpp:325
#8  0x00007f1abdaef487 in KMMessageTagList::prioritySort (this=0x13e4920) at ../../kmail/kmmessagetag.cpp:345
#9  0x00007f1abd89ad97 in KMMsgInfo::tagList (this=0x1c72120) at ../../kmail/kmmsginfo.cpp:312
#10 0x00007f1abd89ae18 in KMMsgInfo::tagString (this=0x1c72120) at ../../kmail/kmmsginfo.cpp:304
#11 0x00007f1abd8e49ed in KMMsgBase::asIndexString (this=0x1c72120, length=@0x7fff57a2ed6c: 148) at ../../kmail/kmmsgbase.cpp:1176
#12 0x00007f1abda31d6d in KMFolderIndex::writeMessages (this=this@entry=0x1031940, msg=msg@entry=0x0, flush=flush@entry=false, indexStream=indexStream@entry=0x100d890) at ../../kmail/kmfolderindex.cpp:526
#13 0x00007f1abda3228f in KMFolderIndex::writeIndex (this=0x1031940, createEmptyIndex=false) at ../../kmail/kmfolderindex.cpp:130
#14 0x00007f1abd90dd1d in KMFolderMbox::reallyDoClose (this=0x1031940) at ../../kmail/kmfoldermbox.cpp:289
#15 0x00007f1abd864b3c in KMFolder::~KMFolder (this=0x1030af0, __in_chrg=<optimized out>) at ../../kmail/kmfolder.cpp:167
#16 0x00007f1abd864ea9 in KMFolder::~KMFolder (this=0x1030af0, __in_chrg=<optimized out>) at ../../kmail/kmfolder.cpp:171
#17 0x00007f1abd8a75be in qDeleteAll<QList<KMFolderNode*>::iterator> (end=..., begin=...) at /usr/include/qt4/QtCore/qalgorithms.h:322
#18 KMFolderDir::~KMFolderDir (this=0xf83cd0, __in_chrg=<optimized out>) at ../../kmail/kmfolderdir.cpp:91
#19 0x00007f1abd8a7619 in KMFolderDir::~KMFolderDir (this=0xf83cd0, __in_chrg=<optimized out>) at ../../kmail/kmfolderdir.cpp:93
#20 0x00007f1abd8a769e in qDeleteAll<QList<KMFolderNode*>::iterator> (end=..., begin=...) at /usr/include/qt4/QtCore/qalgorithms.h:322
#21 KMFolderRootDir::~KMFolderRootDir (this=0xf83b38, __in_chrg=<optimized out>) at ../../kmail/kmfolderdir.cpp:37
#22 0x00007f1abd8aa20d in KMFolderMgr::~KMFolderMgr (this=0xf83b20, __in_chrg=<optimized out>) at ../../kmail/kmfoldermgr.cpp:51
#23 0x00007f1abd8aa2a9 in KMFolderMgr::~KMFolderMgr (this=0xf83b20, __in_chrg=<optimized out>) at ../../kmail/kmfoldermgr.cpp:54
#24 0x00007f1abd967dcd in KMKernel::cleanup (this=0x7fff57a2f4b0) at ../../kmail/kmkernel.cpp:1800
#25 0x0000000000402ff5 in main (argc=<optimized out>, argv=<optimized out>) at ../../kmail/main.cpp:160

Reported using DrKonqi