For a long time now I've often (not always) had DSIs from SimpleMail when viewing a mail folder with a very large number of items (currently 6680 emails). NList.mcc is almost always shown as the cause, and reducing the number of emails typically stops it from happening.
I had thought that upgrading to the latest version of NList (v20.122) had solved the problem, but the problem reappeared when the number of emails increased past some point (perhaps 6000 or so). I have attached a Crash Log from a typical Grim Reaper.
If I ignore the first DSI, then I get another DSI, seemingly without end. I have to tell OS4 to ignore all DSIs, after which I am able to use SimpleMail without any problem! So the bug seems to be relatively harmless - just very annoying.