Note: We do not recommend whitelisting specific servers for allowing emails since their IP addresses could change at short notice and we will add additional servers as the service requires. We cannot guarantee that the information in this guide is complete at all times. The correct way is to check SPF/DKIM/DMARC, since the SPF record will always be updated when any details change.
If you must use whitelisting, and you are experiencing mail deliverability issues, you should send the following list of IP addresses to your IT department to whitelist.
All of these IP addresses are included in the SPF record: spf.ssmx.net