Ran into a scenario where I needed to put an IP address in the Pop3/Imap Server field in fetchmail but kept getting an error: "Pop3/Imap Server is not a valid domain name."
Basically I was replacing an old LAMP server running Ubuntu 8.04 with Ispconfig2 using a new box in the same local area network. After setting up the new perfect server with the same domains, email etc I went to use the fetchmail feature in Ispconfig3 and received the error above. Obviously I could not use the FQDN as it would be pointing to itself instead of the old server I wanted to pull mail from.
My solution was simple: First I set up a fetchmail entry. Then I used PHPMyAdmin to go into the database and modify the record in the table mail_get. Just modify the source_server field and replace the FQDN with the IP address.
After I did this the server started pulling over emails. Hours later I was able to officially retire the old LAMP server.
Hope this helps someone in the future. In the meantime I'm going to enter this on the bug tracker.
Edit: Looks like someone has already added it to the bugtracker. If you need this please vote for it on the bugtracker here