Whitelisting by IP is not recommended. The Cloud Elements platform is a public cloud hosted by AWS, and shared IPs may change without notice; accordingly, the recipient would be whitelisting all traffic from Cloud Elements.
If whitelisting by IP is the only option, the recipient domain will want to check for the most up-to-date IP information by downloading our ip-ranges file. This file will be updated whenever new IPs are added to Cloud Elements' IP range. If a receiving server is whitelisting the range, it's recommended they periodically check to ensure they have the current range.