We're soon going to be making some infrastructure improvements to our API servers. Part of the change will include the IP addresses of our API servers changing.
For most API developers, these changes shouldn't mean you need to change anything. However, if you are using the IP addresses of our current API servers in some sort of firewall whitelist, you will need to make a change.
We recommend you whiltelist api.createsend.com rather than IP addresses from now on.