After opening port with ufw, nmap output shows port as closed

Question

I'm trying to receive SMTP messages on port 25 (this is where my MDA server lives). I've opened port 25 on my box

sudo ufw allow 25

with this output:

Rules updated
Rules updated (v6)

But when I nmap the domain:

nmap -p 25 <my-domain>

I get this output:

Host is up (0.098s latency).
rDNS record for <remote-ip>: hostname
PORT   STATE  SERVICE
25/tcp closed smtp

Why is port 25 closed after I explicitly opened it?

Answer 1

In nmap, a closed port means that the port is reachable, but there is no application listening to the port. Try to run

nc -l 25

(as root of course) and redo the scan.

If the firewall would be blocking the port, nmap would show it as filtered.