The DA version of imapd should be started via inetd. If you are on a Redhat system, they use xinetd and you need to change the setting in /etc/xinetd.c/imapd from disabled to enabled. I don't remember the exact syntax, but if you open that file you will see what I mean. Once you have done that, you need to restart xinetd, you do that via kill -HUP ${xinetd_pid} or I believe you can do an '/etc/init.d/xinetd restart' and that should do the same thing.
If your imapd is already setup in xinetd, then you have to look at the possibility of a firewall blocking port 143.