Public vs Private destination for port 25

[UrbanSage]

Good afternoon,

After initial setup of the firewall I was forwarding TCP:25 to internal mail host.
The rule on the firewall was allow any to "Public IP" from any port to port TCP:25.
This worked for about three weeks and then I started finding blocked access entries to TCP:25
At this point to get it going again I had to allow any to "Private IP" from any port to port TCP:25

Why the change?

[UrbanSage]

Oh, by the way, I have another service for remote access open similar to TCP:25
This rule however is still:
From ANY to "Public IP" from any port to Port TCP:PORT

This is opposite of the SMTP rule.