Our system has detected that this message does not meet IPv6 sending

Solution

The easiest solution is to switch postfix to ipv4 only. Add the following line in the postfix main.cf file and restart postfix:

inet_protocols=ipv4

If you like to use ipv6, then you will have to check 2 things:

1) is there a IPv6 recrd (AAAA) for the hostname "casrv02.myserver.com" in the zone "myserver.com" that points to the IPv6 address of your server?

2) Does your IPv6 address has a reverse dns record that points back to the hostname "casrv02.myserver.com". The reverse recod for the IP has most likely to be set in the ovh controlpanel of the server / vm and not in ispconfig.