Determine why our external public IP for email was listed in a block list provider's listing

curious7 

Whilst many services exist to discover which servers you are blocked on I've found https://mxtoolbox.com/blacklists.aspx to be very useful, then you can contact each list provider to see why they blocked your IP, they all have a tool where you put in your IP and the generic reason is shown... (sent spam / open relay etc)

Its worth also checking the reputation of your IP after an incident like this as it may also cause residual issues even once you have resolved the problem, sites like -  https://www.cyren.com/security-center/cyren-ip-reputation-check provide one such reputation checking service... (other companies are available.)

In terms of knowing where the problem came from, do you have logs from your External firewalls, this would hopefully show a lot of SMTP traffic ?