AOL error when sending invoices

When emailing an invoice through Books I get en email like:

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This is a MIME-encapsulated message

--q8OEAfcM022819.1348495841/resolver1.sv4.zoho.com

The original message was received at Mon, 24 Sep 2012 07:10:39 -0700
from [172.31.255.33]

----- The following addresses had permanent fatal errors -----
<edea8888@aol.com>
(reason: 554- (RTR:BL) http://postmaster.info.aol.com/errors/554rtrbl.html
)

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According to AOL that means:

This error message indicates that a permanent block has been placed against your IP due to poor IP reputation. Apply for a complaint feedback loop before opening a support request .

Can someone from Zoho explain what's going on?

Thanks