Ok I've clicked the "Click Here to Find Similar Threads" button, but none of the other threads had been answered! There did seem to be a lot of them though, meaning that I'm probably not the only one with a problem.

I'll just do this from scratch, I downloaded mixmaster via apt-get (on Ubuntu 8.10):
and that worked perfectly like always, then I typed 'mixmaster' and it opened. I started by selecting "u)pdate status" and hit the '*' to update from a list of pingers and selected 'b' for 'banana'. Then I selected "m)ail" and did the following
I started editing the message by selecting "e)dit message", got into a vi editor, selecter "i" and wrote the body and then <esc> and ":wq". Then I selected "m)ail message" and finally "s)end messages from pool" but mixmaster gives the following error message:
I've tried installing both mixmaster and postfix and removing them, and installing again but to no avail. Sometimes the error message doesn't appear but I don't receive an e-mail at "myown.address@gmail.com" nevertheless. Why doesn't this work??

Please note that it can take a long time before the message is delivered ( up to one day or more )
Any pointers in the logfile from mixmaster?

I didn't know that it could take such a long time to send it.. (I thought that might be the problem because mixmaster sends to 3 clients [I think that's what they're called] by default and each of them has to decode the message if I'm not mistaken)

Where is the logfile located?

/var/log/mixmaster/err.log

Hm, the /var/log/mixmaster file is empty on my computer.
Also, I sent around 8 e-mails that were 'successfully' sent I they still haven't delivered- that was almost two days ago. Is it normally this slow? Why?

Do you see if the mail is sent in the logfiles from your mailserver?
Do you have problems sending normal mails using postfix?