Florian,
Before I begin typing out the reason of this post, let me just start off by saying that less than 24 hours ago, I knew absolutely nothing about Unix systems, barely knew that Python was a programming language and was still trying to figure out exactly what Fetchmail is (the latter still applies tbh).
After a lot of googling, a ridiculous amount of frustration and even more nail biting, I FINALLY got it to work. Turns out that the version of Fetchmail that comes with OS Lion Server is 6.3.18, has a known bug. Therefore, it produces the following error:
/usr/bin/python: can't open file '/fetchmailconf.py': [Errno 2] No such file or directory
Google found me a lot of bug reports on this issue. However, all bug reports were on a variety of different Linux systems where the files are located in different folders (as far as I could understand). Anyway, To overcome this error, I simply downloaded the latest version of Fetchmail, took the file "fetchmailconf.py" and placed it in /usr/bin/ on my computer. After running fetchmailconf (not to be confused with the one ending with .py) I got another error:
Traceback (most recent call last):
File "/usr/bin/fetchmailconf.py", line 2103, in <module>
hostname = socket.gethostbyaddr(socket.gethostname())[0]
socket.herror: [Errno 1] Unknown host
Even more googling and I discovered that it has to do with the local host not being set because I'm on a private network.
Fix: Go to /etc/hosts and add your local (internal) IP address.
Run fetchmailconf again from Terminal and you're good to go.
Should you have any problems, drop me a line on niklas [att] garphult dot com.
Good luck!