The usual reason for ending up with a queue full of bounces is that you're accepting incoming mail for non-existent users: for example, you accept incoming mail purely on the basis of the domain name, without checking the local part. Once Exim has accepted the message, it then tries to deliver the message, and only then does it find that the message is undeliverable. The message therefore bounces. If the bounce message cannot immediately be delivered, it will stay in the queue.

First, ensure that your routers reject unrouteable addresses. Use exim -bv to test that "good" addresses, including external ones, are routeable, and that "bad" addresses are unrouteable.

Then, ensure your acl_smtp_rcpt includes "verify = recipient".

To actually get rid of bounce messages already in the queue, try one of these methods:

exipick -f '^$' -i | xargs exim -Mrm

or, equivalently:

exipick '$sender_address eq ""' -i | xargs exim -Mrm