One GCC dev told me once that no app should ever crash no matter what is the input. ...and while most crashes are harmless zero-pointer dereferences some crashes are exploitable => if there is any chance you can send the devs the mail that caused the crash, perhaps after saving it as .eml an compressing it into a .zip file the devs will most probably be grateful: Most crashes are easy to fix once a dev can reproduce them because the debugger will most probably be able to tell which line the crash was caused by.
Unfortunately, as Schneier says security is a moving goal: every day bugs are fixed and every day weird contents is created that triggers bugs and even simple things that used be near-plain text (like mails and websites) get more complicated every day which means each app will be "crappy" regarding to your definition from time to time. The cause for the crash might even lie completely outside if the app in a library the phone happens to provide. In this time it cannot be found in k9's code nor using any different phone than a S9. I am not a k9 dev. But with wxMaxima, the program I support I have learnt to dread this kind of bug.
--
Diese Nachricht wurde von meinem Android-Gerät mit K-9 Mail gesendet.