Mailing to an external audience(public)
The number of companies using security software is growing rapidly. This type of software is aimed at preventing impersonating email where the from address and the server it comes from do not match. This protection against spoofing use methods such as Sender Policy Framework (SPF, analogy of microsoft Sender id) or domainKeys identified mail (dKim).
Further information on these technologies can be found here:
When you’re going to send email from WorldAPP’s servers to your own domain (i.e. to your own colleagues), especially if these will be made to look like a genuine internal email address @ your domain, then it is strongly advised to get WorldAPP’s server whitelisted. This requires authorising access to your mail server for emails coming from the following IPs of WorldAPP servers: 220.127.116.11 through 18.104.22.168.
For more information on DKIM and SPF records set up, please refer to this help page.