How can I check if a relay is blacklisted (RBL)?

RBL (Real-time Blackhole List) checks should be done in filter_begin or filter_sender to reject blocked senders as early as possible. Use relay_is_blacklisted for a single RBL or relay_is_blacklisted_multi_count to query several RBLs simultaneously and reject based on a hit count threshold.

use Mail::MIMEDefang::Net;

sub filter_begin {
    my($entity) = @_;

    # Single RBL check
    my $result = relay_is_blacklisted($RelayAddr, 'zen.spamhaus.org');
    if ($result) {
        md_syslog('Warning', "Relay $RelayAddr listed in zen.spamhaus.org ($result)");
        return action_bounce("Mail rejected: $RelayAddr is blacklisted");
    }

    # Multi-RBL check: reject if listed in 2 or more RBLs
    my @rbls = qw(zen.spamhaus.org bl.spamcop.net dnsbl.sorbs.net);
    my $hit_count = relay_is_blacklisted_multi_count($RelayAddr, @rbls);
    if ($hit_count >= 2) {
        md_syslog('Warning',
            "Relay $RelayAddr listed in $hit_count RBLs - rejecting");
        return action_bounce(
            "Mail rejected: sender IP listed in multiple blacklists");
    }
}