15:37:15.971: >> 554-Bad DNS PTR resource record.<cr><lf>
The arrows pointing to the right denote the remote server's response/query (arrows pointing to the left are your server's responses/queries).
As Rolf says you need to ask your ISP to set a PTR (PoinTeR) record (a reverse lookup record which the opposite of an A Record).
As you are probably aware a DNS query for a name requests the A record so that the name (e.g. google.com) can be resolved to an IP address. A reverse record, known as a PTR record allows you to resolve an IP address to a name. Mail servers use it as a way to ensure the name the server announces itself as matches the IP address the mail message originated from.