When upgrading from Tiger to Leopard without doing a clean install, there always a chance that the sort of thing you're experiencing with mail can occur.
You can try to completely remove mail (including the left overs from Tiger) and then reinstall it. However, you'll need a third party utility to extract mail from the Leopard DVD without having to reinstall everything along with it.
Pacifist from Charlessoft will do it for you.
CharlesSoft - software you always wished someone would write
Regards.