Problem

What are email block lists?
What are email black lists?

Solution

Most Internet Service Providers (ISPs) and web servers use a collaborative service to help block and prevent spammers from doing their business.  These block lists (as known as black lists) are DNS based anti-spam databases.

Your IP Address might have changed, resulted in you being blocked. Or it maybe that you IP address has been flagged for some reason and blocked by one or many of the DNSBL (Domain Name System Black List) services, but why?

Assuming you are not a spammer of course, the most common cause is your PC (or a PC in your network) has been infected with either a virus or malware.  The infected computer has then been sending spam (junk unsolicited emails) on behalf of the spammer.

Another reason your IP could be blacklisted is through servers or routers being misconfigured to allow unauthorised users to send email through your ISP. These security holes allow what is called an Open Relay, and the spammers constantly search the Internet for vulnerable systems to use.