This is a rather complex problem but I think sendmail is the place to fix it. If you use (vixie-) cron (at probably as well) and the outputs of your jobs include non-ASCII characters (e. g. umlauts), they are not declared and therefore cannot be displayed properly in the MUA. Instead of tweaking cron, at and the dozens of other components who make use of sendmail, I would suggest to change latter's behaviour: *If* there is no "Content-Type" header *and* there are characters > 127 in the input, *then* add a "Content-Type: text/plain; charset=utf-8" header.
This is not something I would change for Red Hat Linux. please talk to the upstream sendmail people if you want to get this in. greetings, Florian La Roche