Thank you, mow. I tried both mail-tester.com and a gmail address that works. These are the results.
mail-tester.com
# tcpdump -v port 25
tcpdump: listening on venet0, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
04:13:00.417337 IP (tos 0x0, ttl 64, id 5443, offset 0, flags...