telnet localhost 25

telnet validip 25

tcp6       0      0 :::25                   :::*                    LISTEN

$ nmap validip
PORT    STATE  SERVICE
25/tcp  open   smtp

# ufw status
Status: active
To                         Action      From
--                         ------      ----
25/tcp                     ALLOW       Anywhere
25                         ALLOW       Anywhere
25/tcp (v6)                ALLOW       Anywhere (v6)
25 (v6)                    ALLOW       Anywhere (v6)