Unable to access certain sites in mac
This post from the Apple Support Communities might help: Link

Things to try according to this post:


Settings/Networks/Advanced/Proxys, Discover new proxys
Check the ignored domains for blocked domains
Safari/Preferences/Advanced - enable the Develop menu, then empty caches
Safari/History/Show History - delete all history items
Restart your router (obvious one, but often works well)


One other thing to try:
 - Change the DNS server to Google: 8.8.8.8, 8.8.4.4