[anti-abuse-wg] What to do if ISP rejects Abuse Reports?

Hello,
my systems get attacked from several IPs from Georgia (Countrycode=GE, RIR=RIPE).
My Abuse Reports to the abuse address (ib at caucasus.net) of the
responsible ISP for the attacker IPs (caucasus.net) just bounce:

#ID:        <SCC9B>
#Mail From: <security at mutluit.com>
#Rcpt To:   <ib at caucasus.net>
#Server:    <mail.caucasus.net> [62.168.168.131]
#
#[<02>] The reason of the delivery failure was:
#
#550 5.7.1 <security at mutluit.com>: Sender address rejected: Blocked by postmaster


What to do in this case?