I don't know how it landed on my server (maybe it was apt-get install mail), but:

shephard@sbwc:~$ cat /etc/issue

Ubuntu 8.04.2 \n \l

shephard@sbwc:~$ sudo netstat -anp | egrep -i tcp.25

tcp 0 0 127.0.0.1:25 0.0.0.0: LISTEN 21313/exim4

Ubuntu seems to have an affinity for exim somewhere...