Full Error Message:
A5 Fax App could not register your Salesforce Org.
Your org may be locked out for external API calls.
Contact Support for further help.Attempting to register the following Org ID: 00Dt000000XXXEAI
Solution:
Customer who receives this error by email should contact Salesforce support (or their Account Executive) to have them unlock external API calls. Also ensure that API is enabled in Salesforce org and has not reached the daily API call limit.