Are you saying that you didn't tell him a certain amount of emails would be mailed/inboxed for the payment he sent you? Did you or did you not meet the quota of mailing/inboxing that be paid you for? Simple question, needs only a simple answer.
Someone here gave the analogy that when you hired someone to cut the tree, and the axe broke, its not the tree owner's fault.
We discussed lengthily about the mailing process, and we clearly agreed on malling company profiles. The creative that he gave me was clearly "pump and dump".
In other words, we agreed upon on the type of tree that would be cut, and I clearly told him the type of tree that I would wish to cut and the tree that I would not be willing to cut.
He then delivered a completely different tree, and the axe broke while in the process of cutting the tree.
Should he be charged for the axe?
What about the opportunity cost of the broken axe? Business lost, emotional stress, etc.
If he wants me to complete the mailing with the broken axe, I have no qualms about it. We didn't agree on inboxing, just the mailing rate. We agreed on mailing with one specific axe, and it is already broken.
Mailers would know that there is no point of mailing emails with blacklisted IPs, as it will just land in the spam box or not get delivered at all. The open rate for emails in the spam box is too low for any realistic conversions.