After killing frozen GNUMail in the middle of a message move operation -- which may or may not be relevant -- it crashes on startup with this message:
GNUMail: Uncaught exception NSGenericException, reason: Glyph generation was triggered for a layout manager while the text storage it was attached to had unprocessed editing. This is not allowed. Glyph generation may be triggered only at points where calls to -beginEditing and -endEditing are balanced
I don't remember how to debug. "debugapp GNUMail" does not provide a stack trace.