Exim ACL To Send Mailer Daemon Messages To ":fail:"

One of my users is receiving way too many Mailer Daemon messages and his mailbox is full. I've had this problem from time to time and I am trying to figure out how to block mailer daemon messages for a specific domain so that they do not even get on the mailing queue...much like when you set a default address to ":fail:". So I came up with this:

refuse_md1:
deny message = The original message did not come from this site.
condition = ${if eq{$sender_address}{}{yes}{no}}
condition = ${if eq{$local_part}{userdomain.com}{yes}{no}}
log_message = Refused a bounce message for userdomain.com

However, this doesn't help. The emails are still going to the mailing queue and when I look at the Exim log I see the usual error message saying that the email was blocked because the account has run out of space.