With netstat and -p option (the pid may not be shown if the command is not executed from the user which runs the process):

venus:~# netstat -tlnp | grep -w 25
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      2931/tcpserver  
venus:~# ps wwu 2931
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
vpopmail  2931  0.0  0.1   5912   564 ?        S    May02   0:00 /usr/local/bin/tcpserver -v -R -l gz.ro -x /etc/tcp.smtp.cdb -c 30 -u 1016 -g 1011 0 smtp /var/qmail/bin/qmail-smtpd mail.gz.ro /home/vpopmail/bin/vchkpw /usr/bin/true

With netstat and rmsock:

root@gz~# netstat -Aan|grep 25 | grep LISTEN
f100060003582398 tcp4       0      0  *.25               *.*                LISTEN
root@gz~# rmsock f100060003582398 tcpcb
The socket 0x3582008 is being held by proccess 385234 (smtp).
root@gz~# ps -ef | grep 385234 | grep -v grep
   notes  385234   78038   3   Nov 06      - 15:08 /opt/ibm/lotus/notes/latest/ibmpow/smtp

With lsof:

root@gz~# lsof -i :25 | grep LISTEN
smtp    385234 notes   10u  IPv4 0xf100060003582398      0t0  TCP *:smtp (LISTEN)
root@gz~# ps -ef | grep 385234 | grep -v grep
   notes  385234   78038   1   Nov 06      - 15:08 /opt/ibm/lotus/notes/latest/ibmpow/smtp

With socklist. In socklist the second column is showing the port and on 5th you have the pid:

root@gz~# socklist | awk '{if($2=="25") print $0}'
tcp     25       6930       0   385234    3  notes
root@gz~# ps -ef | grep 385234 | grep -v grep
   notes  385234   78038   1   Nov 06      - 15:08 /opt/ibm/lotus/notes/latest/ibmpow/smtp