I found the answer to this. ListenSMTP can sit behind LB if the processor will run on all nodes if single DNS is to be exposed. you can also have processor only run on single node (primary).
Another option, not tested or fully vetted. have SMTP response payload into nifi rest processor. from there push to process group which will have SMTPListen processor. This will automatically load balance
@Artem Ervits I provided this answer after some research. no response from other. please accept if this answer is good.