1. What is the pattern of recipient email addresses for these "failed" forgotten-password emails ?

2. Since you host it, perhaps you can monitor whether the forgotten-password emails ever get to your mailserver. There's no reason why they wouldn't. Chances are something "in" the email or at the recipient's host mailserver is what is preventing it from being delivered.
Is your SpamAssassin or equivalent tool dumping the emails for some reason ?