Network Cards failing

I’m using Ubuntu Server and Exim 4 as a simple SMTP server for a medium sized business. I have a network card for the public network and the private network. It seems that whenever the start getting hit hard that they shutdown, the Nmap results go from open to filtered but it’s only on one nic, the other if I scan it locally they still show as open:

zach@mail:~$ nmap localhost
25/tcp open smtp

zach@mail:~$ nmap publicip
25/tcp filtered smtp