DataPacket's shared Web hosting platform uses RBL lists to determine the reputation of a mail server. Mail is blocked when the sender's mail server IP address is block-listed for having recently sent spam or other abusive mail.
You can run a sender's mail server IP address through an RBL lookup service to see if it would be blocked, one is provided below.
Generally, in a case when mail is blocked, the sender would contact their provider for help. The provider would go through the process of stopping the abuse and working with RBL providers to have their IP cleared from the block list.
RBL Lookup - http://multirbl.valli.org/
Our shared Web hosting systems use the following block-list providers.