In an attempt to fight spam, several organizations began building black lists in
order to ban domain names or IP address ranges that were accursed to be involved
in unwanted practices. Many large organizations use these block lists to filter
incoming emails. If you notice that some of your emails consistently fail to reach
specific recipients and you verified the email address for validity, check if your
IP address is blocked and, if so, talk to your provider to rectify the situation.
If this happens more than once, get yourself another, better provider.