When this happens, please try to open the link yourself.
If you can successfully open it, the invoice is probably blocked by a firewall on the client side. Please advise them to whitelist the IP addresses below.
109.68.163.0/26
109.68.163.128/26
109.68.163.192/26
109.68.167.96/27
A large chunk of our traffic comes over 1 IP address: 109.68.163.183 (109.68.163.183/32)