Question : Reverse DNS failing

We've revently changed ISP so our IP addresses in each of our sites have changed; therefore the fixed IP of our mDaemon mail server has changed also. Some staff are getting delivery failure messages and it seems the cause is that the reverse DNS lookup is failing.  I don't see why though, as the correct IP is set up when I check using mxtoolbox.com.

It is also worth noting that the firewall has changed now from our own firewalls in each site to a group firewall operated by our ISP's managed services.  Might there be a port on the firewall which I should be opening for reverse dns lookups to work properly?

Answer : Reverse DNS failing

Keep in mind not only do you need reverse DNS (PTR record) for your IP, that name needs to resolve back to that same IP. If not, it just looks forged and you'll get rejected also.

1.1.1.1 -> mail.yourdomain.com AND
mail.yourdomain.com -> 1.1.1.1