I am in need of help trying to figure out why I keep getting put on a blacklist every couple of weeks with The CBL and now sophos.com

"Your IP has been HELOing as using the string
;; connection timed out; no servers could be reached".

I have researched and researched but I am unable to find out what this means. My zimbra server is up to date patch wise
I have a firewall in place
I am not an open relay
I am totally confused...has anyone else had this problem and figured out how to correct it?