We try very hard to NOT block good email but it does happen, and when it does we try very hard to resolve it as quickly as possible.

The reason good email gets blocked predominantly has to do with IP addresses. IP addresses are how all the devices on the internet recognize one another. When you have a machine connected to the internet you may know that machine by name but other machines on the internet know that machine name by and IP address.

An Internet Protocol address (IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing. http://en.wikipedia.org/wiki/IP_address

If you have an IP address that has had bad behavior coming from it in the past it may be blocked for the predecessor behavior.

If you are on shared services it may be someone else behaving badly on the same network.