Obscure problem: forwarded mail bouncing

[codesmith]

We have our domains set to reject mail if the user doesn't exist. However, one person has an email set up (example aaa@mydomain.com) to forward to another email address (example bbb@otherdomain.com).

Say a spammer sends an email to aaa@mydomain.com from ccc@baddomain.com.

Our server mydomain.com tries to forward the email but otherdomain.com has better spam controls and rejects the bad ccc@baddomain.com email address. Our server mydomain.com now tries to bounce the email back to ccc@baddomain.com which we don't want to happen. Any ideas on how we stop this bouncing?