[Exim] Ignore freezes due to unknown user in 4.40?

Is it possible to simply ignore messages that are a result of "unknown
user"? Someone just tried over 2000 @mupp.net mail adresses, and well, i
now have 1999 messages frozen (They actually DID manage to hit one existing
user)

Jul 21 13:53:38 statler exim: 2004-07-21 13:53:38 1BnFfE-0007zZ-QJ **
solicitor@??? R=scooter T=remote_smtp: SMTP error from remote mailer
after RCPT TO:<solicitor@???>: host scooter.mupp.net [213.212.33.23]:
550 Unknown user
Jul 21 13:53:38 statler exim: 2004-07-21 13:53:38 1BnFfE-0007zZ-QJ Frozen
(delivery error message)