that site is not even categorized so content filtering won't block it. Just make sure you are not blocking the URL somewhere or blocking the site IPs (18.104.22.168 and .158 / 22.214.171.124 and 145). Just to make sure you didn't miss anything, just whitelist your computer, wait about 5 minutes and try again.
If the site is site blocked, try with a different DNS like 126.96.36.199, 188.8.131.52, and 184.108.40.206. If still doing the same thing, this could be something upstream blocking the site especially if your home router is in NAT mode.
Also, this could be something to do with your home public IP that was blocked on the server-side. If the MX is using a static IP, bypass the MX and connect your computer directly to the ISP router using the same static IP used by the MX and give it a test.