Mail says Mail Attachments Iconizer may be responsible for a crash.
When Mail crashes on Mac OS X 10.5, it tries to guess whether a plug-in was responsible for the crash. If it thinks so, it will warn you that Mail Attachments Iconizer (or another plug-in) may be responsible for this crash.
Note that these warnings do not mean that the plug-in in question really is responsible for crashes.
That said, if the situation reoccurs or if you suspect Iconizer to really cause the crashes for some reason, check that you have the latest update and send us the crash report files. They are located in your home folder in Library/Logs/CrashReporter and their name begins with Mail.
Both updates and contact information can be found at http://lokiware.info.