Exim uses a block list zen.spamhaus.org to fight against spam but spamhaus doesn’t allow public resolvers querying their DNS servers so, seems you are using a public resolver like 1.1.1.1.
Options to solve the issue:
1.- I don’t recommended this option. Remove zen.spamhaus.org from /etc/exim4/dnsbl.conf and restart exim systemctl restart exim4
2.- Use your own dns resolver or use one that is not blocked by spamhaus.
If you want to test whether your current dns resolver will work, execute this command:
dig (echo '212.227.15.15' | awk -F '.' '{print $4"."$3"."$2"."$1".zen.spamhaus.org"}') +short
If you see nothing, all is good, your dns resolver is not being blocked. If you see an output like 127.255.255.254 then your dns resolver is blocked by spamhaus.
If you want to test whether a resolver will work, add @dns_resolver_ip to the dig command, example using 1.1.1.1.
dig @1.1.1.1 (echo '212.227.15.15' | awk -F '.' '{print $4"."$3"."$2"."$1".zen.spamhaus.org"}') +short
3.- If you don’t want to install your own dns resolver or use another public resolver and want to keep your current one, you can request a query key from spamhaus (it is free) and the doc to request and use it is here: