Email IP Address for Whitelist

I am working on getting maildelivery@invoiceninja.com whitelisted. What is the IP address that emails come from? I cannot seem to find it anywhere.

We use Postmark to send our emails, from the link below I believe the IPs are 50.31.156.96/27 (50.31.156.96 - 128)

http://support.postmarkapp.com/article/800-ips-for-firewalls