When i just looked up a domain, it said that a reverse DNS lookup does not work for our IP, i suspect this is the reason why we are getting some specific undeliverable emails. Does any body know how i configure the DNS to allow a reverse lookup to work ? Cheers

Hello : found this, looks relevant or :) Good Luck Theo How to fix the problem Callers experiencing reverse DNS lookup failures have two options: either to "unhide" their workstation's IP address -- by adding it to the DNS computer's address list -- or to call from (different) workstation whose IP address will be verified by its official DNS computer. Most callers cannot on their own add IP numbers to the official DNS computer address lists, nor can we. That change must be made by the DNS computer operator, typically the callers' internet service provider or technical support personnel at a workplace. Callers should ask them to enable "reverse DNS lookups" for their workstations.