Ok - I found the test Sagepay at: https://test.sagepay.com and I entered the Infusionsoft IP addresses into the page. Then I went to Test Account in Infusionsoft and received the same error:

Error details: 4020 : Information received from an Invalid IP address.

The IP addresses we have entered in Sagepay (both test and live) are:
063.226.035.128 (255.255.255.128) -
067.131.025.000 (255.255.255.192) -
069.007.030.000 (255.255.255.000) -
072.009.198.096 (255.255.255.224) -
072.009.221.048 (255.255.255.240) -
072.009.221.160 (255.255.255.224) -
072.166.076.064 (255.255.255.224) -
208.076.024.000 (255.255.255.000) -
216.027.233.096 (255.255.255.224) -

Any other ideas? Thanks so much! This is extremely frustrating (especially for the client). Sagepay works fine for another system they are using - just having problems with Infusionsoft.

Thanks again.