Weird mail routing

I've been working on my mail system these day. Everything works fine, except that e-mails generated locally (usually from cron to root) end no longer in /var/mail/root but go to /var/spool/mqueue where they stay as they are refused by the provider's mail-server. Outgoing e-mails are processed OK. Where should I look ? Thanks, -- ~adj~ Ces mystères nous dépassent Feignons d'en être l'organisateur...