If you had installed A5 Fax prior to February 2019 you must update your A5 Fax app along with the url/domain it points to following the steps below.

1. Update App from AppExchange
2. In Salesforce, go to Setup-> Security -> Remote Site Settings
3. Click edit on “Fax Service”
4. Change “Remote Site URL” from “https://fax.ramseysolutions.com” to “https://fax.a5faxapp.com“, and Save.