The cause of the problem is that some ISPs have mistakenly added Prohashing to an E-Mail blocklist on their mail servers. One of the biggest offenders is att.net, which rejects all Prohashing messages. All customers with an address ending in "@att.net" will be required to enter new addresses, as messages to that domain are not being received anyway. If you are affected, you can feel free to submit a support ticket at att.net, and request Prohashing's removal from their list, and we'll be glad to remove the restriction. There are some smaller ISPs that are also affected - mostly from community colleges and a few universities.
We will be adding other addresses and domains to the blocklist to reduce the number of support tickets we receive. Again, this change will not have any actual impact on customers, because customers using domains that are being removed were missing important mail we were sending them anyway. Feel free to submit a ticket to us once you have submitted a ticket to a domain that is on our list, and we'll be glad to remove it after the receiving domain takes action to remove us.