Interested to know what IP Blacklist monitoring services you guys have tried/use to check if your customers (or your own) IP's get blacklisted?
MXToolbox is good, but at $20USD a month, pretty pricey.
I use HetrixTools. They have several plans, free plan allows up to 32 IP's, monthly plans from $9.95.
I've also been playing around with self monitoring using BlackListMonitor.