When mails are sent to some users on a LAN network system,they are not delivered.After some troubleshooting,i discovered that it seems like their system is blocking incoming mails.Also,some websites dont open on some particular system.What do you think is the problem and how do i get over this?

disable proxies (unless the setup needs them), check firewalls.

proxy can be one reason and the config of your firewall can also be another. if there is no proxy configured on the network, then check your router. this is whre the issue usually is.

confirm by checking whats the sites have in common , that makes them not open on some systems, problem solving basics