ONLY when we include the zen.spamhaus.org RBL in the “Realtime (SBL) Blacklists:” section of our incoming mailserver is when we get a “timeout” when trying to connect. For some reason, zen.spamhaus.org doesn’t seem to be working on this server but on others configured exactly the same it works like a charm!
A few very quick things that I would check:
Can you ping it?
Can you resolve it?
Can you do manual blacklist lookups from the server using the ‘host’ or ‘dig’ commands?
Has zen.spamhaus.org been entered in /etc/hosts?
Has its IP been entered in /etc/hosts.deny?