Not only Open DNS/Umbrella or Firewall does this, but also Mimecast sometimes blocks websites as well. You need to confirm with the recipient if this is a known URL for them and check it on Virustotal and Silo/Auth8.
See ticket as an example: https://mayfieldhelp.freshdesk.com/a/tickets/7000
Login to Mimecast then go to Administration >> Services >> URL Protection
Search for the blocked domain, and click on Allow URL
Give it a couple minutes and test it.
Thanks,
Audel