If the link came through some webmail the link may be through a bounce. Gmail and such although they display the real link have redirection underneath to record links hit by their users, something like
https://googlecrap.com/%https::....obarun.org and some systems or isps may view this as a potential hazzard, phising, whatever they call their security crap, and may not have obarun in some whitelist. Tell them to copy the address and paste it directly on the browser to see if it opens. If it doesn't then their dns server is blocking it.
We got about 30-40 years of free net, it is all downhill now. Read 1984 thinking it is 2018
Or the server is running on a cloud and their isp is being blocked?