Your application must be able to reach Checkbook's domain names for it to function properly and depending on how your integration operates, you may need to whitelist them.
To help your integration operate securely, it must also verify that it’s communicating with our servers through one of our IP addresses listed below.
Production (api.checkbook.io)
99.83.192.53
75.2.12.84
Sandbox (api.sandbox.checkbook.io)
76.223.87.18
13.248.217.122
If your integration also receives webhooks from Checkbook, you should ensure these events originate from one of our IP addresses (the same for Production and Sandbox).
Webhooks IP addresses
52.10.180.255
54.70.79.20