Denied by Policy. TLS conversation required

Hello everyone. When a external partner is trying to send us an email he is getting an NDR saying that Denied by policy. TLS conversation required. However accept TLS connection is already enabled under inbound configuration is scanner settings. All the other domains can send us fine over TLS. This is happening with only one domain that is trying to send to us. Is this issue on my side or their side and what needs t o be done to fix this?

My SMG scanner is mail3.saib.com.sa