I couldn't find any documentation other than a 1-page glossy on the DX-1000. The document did mention that it was a POP server (as well as an SMTP server). Do you know if people are deleting their messages from the server as they download them?
You can test to see if there are a lot of messages on it by doing the following:
telnet <panafax IP> 110
You should receive a banner
USER <username>
+OK
PASS <password>
+OK
STAT
+OK <number of messages> <length of messages>
QUIT
+OK
Connection closed by foreign host.
Ideally, on any POP server, the answer to STAT should be '+OK 0 0' which means that there are no messages to pick up. If you get something else, especially if either number is large, then you should start deleting downloaded messages.
pansophic