ConfigServer Firewall has been great at keeping unwanted users at bay on my web servers, but it also causes plenty of problems – notably when the SMTP authentication fails multiple times. This is usually caused by an email account trying to send an email and repeatedly using bad credentials to login. After a short period of time, CSF or cPHulk kicks in and blocks all communication with their website.
Managing this is a pain as there is no way (as far as I can tell) to allow clients to white list themselves when this happens. So, I’ve put up a page on a different server that handles requests for white listing.