Zix Secure Message Portals will use a variety of IP address:
The Secure Message web sites are typically in the below IP range:
63.71.13.0/24
63.71.14.0/24
63.71.15.0/24
199.30.236.0/24
91.209.6.0/24
Notices sent from the portal will typically come from smtpout.zixmail.net, (which is IP addresses 63.71.8.100 through 63.71.8.109) but can also come from one of the above ranges as well.