I got the following error while sending emails to a particular customer:
518.104.22.168 Connection rejected. Reverse DNS lookup failed. Sending IP address does not match existing 'A', 'AAAA' or 'PTR' record. Connection to host lost.
I do have two Reverse DNS records set up in my ISP:
The A record for my mail server is :
Can you tell what causes my mails being rejected?