I’ll keep adding some good reference material here for checking Linux connections from the command line:
yum install tcpdump
See all port 25 connections
tcpdump -n dst port 25
tcpdump -n dst host * port 25
tcpdump -n dst host any port 25
tcpdump -i eth1 tcp port 25
Go to the link above if you’re looking for a way to view Apache logs in real-time.