If you're being deluged by backscatter email, there is a way to block at least some them with Exim using a DNSBL. However you need to treat these sources differently from normal spam sources.

A database of backscatter IPs is available for use via backscatterer.org but, as it warns, you'll want to use it in "SAFE" mode.

Firstly, if you don't already have one, you'll want to add a local ACL file for the RCPT ACL check. On a split config, add something like the following to /etc/exim4/conf.d/00_local_config .

CHECK_RCPT_LOCAL_ACL_FILE=/etc/exim4/local_acl_check_rcpt

Then edit this file, or your local equivalent, and add the following:

deny senders = :
     dnslists = ips.backscatterer.org
     log_message = $sender_host_address listed at $dnslist_domain
     message = Backscatter: $dnslist_text

The trick here is the senders line contains a single colon, which will match the NULL sender used by the vast majority of bounce sources.

If you want to test it out before activating a deny rule, use a warn rule to begin with:

warn senders = :
     dnslists = ips.backscatterer.org
     log_message = $sender_host_address listed at $dnslist_domain
     message = X-Backscatter: $dnslist_text

Update the config with update-exim4.conf and restart the exim daemon to activate.

Note: mail to postmaster is, by default, not affected by locally applied ACLs on a standard configuration. You'd need to make additional changes if you want to block backscatter sources from mailing postmaster - but this is not advised.