By default EXIM4 only warns about mails with no reverse DNS. I need to block and actively reject mail which has no reverse DNS. I am willing to accept that it's possible that some legit mail will be blocked, however, so far the only mail with no reverse DNS has been spam.
Below is the code for reverse DNS checking from the EXIM4 config. How do I change this from warn to blocking.
# Warn if the sender host does not have valid reverse DNS.
#
# If your system can do DNS lookups without delay or cost, you might want
# to enable this.
# If sender_host_address is defined, it's a remote call. If
# sender_host_name is not defined, then reverse lookup failed. Use
# this instead of !verify = reverse_host_lookup to catch deferrals
# as well as outright failures.
.ifdef CHECK_RCPT_REVERSE_DNS
Reverse DNSReverse DNSwarn
condition = ${if and{{def:sender_host_address}{!def:sender_host_name}}\
{yes}{no}}
add_header = X-Host-Lookup-Failed: Reverse DNS lookup failed for $sender_host_address (${if eq{$host_lookup_failed}{1}{failed}{deferred}})
.endif