Do you have a list of ClickSend server IP addresses we can Whitelist?
When connecting to our API
Unfortunately we have many servers where the IP addresses are subject to change due to load balancing. The best method would be to whitelist our hostname: rest.clicksend.com and api.clicksend.com
When we send web hook/HTTP posts to your server
There is an option for us to post delivery reports and inbound messages to your web server. These are the IP addresses that we use: