nullmailer not listening on port 25

nullmailer is described (in the deplorable incomplete manpage) as a slim MTA. How can it be that it is a MTA without listening to the smtp port TCP-25? Is this a malfunction/misconfiguration on my machine?
hhl@koha-VM-play:~$ uname -a
Linux koha-VM-play 3.13.0-48-generic #80-Ubuntu SMP Thu Mar 12 11:16:15 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
hhl@koha-VM-play:~$ sudo netstat -anp | grep :25
tcp 0 1 172.26.195.11:35112 198.18.17.16:25 SYN_SENT 3709/smtp
hhl@koha-VM-play:~$ telnet localhost 25
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
hhl@koha-VM-play:~$