Check result of IP188.8.131.52
This is the rDNS found:
This IP was flagged due to misconfiguration of the e-mail service or the suspicion that there is no MTA at it.
For the delist key can be sent, select the e-mail address responsible for this IP:
The rDNS must be registered under your own domain. We do not accept rDNS with third-party domains.