In that case I would recommend sending test emails to other accounts on other systems that don't bounce ( ;) ) and carefully inspect the "reply-to:" and "from:" headers to make sure they are what you think they are.
I currently work for a company that regularly get determined to be a spammer (I really think we're not, honest!) and it's always a careful read of the bounces to determine who has us (and more importantly why we are) blocked.
It's possible that by directly interfacing with sendmail that the "From:" header of your sent emails are showing up differently? Or perhaps you have email aliases (/etc/aliases) sending that mail to that account directly to /dev/null? Perhaps a .forward file you weren't aware of? Also try sending a test email to your bounce handling account to make sure you can receive at least that :)
s**lil*; $*=join'',sort split q**; s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)