In order to make sure I did'n have any typos, I did a complete reinstall (copy/paste) from the tutorial. I now have these problems:
I still get the no dir exists warning when trying to log in, but mail sent from my GMail account doesn't bounce but times out:
Technical details of temporary failure:
TEMP_FAILURE: Could not initiate SMTP conversation with any hosts:
[mail.sailmaster.nl. (5): Connection timed out]
Mail sent to the second domain I set up bounces with the previously mentioned relay error
(I have entered 2 domains in mySQL - sailmaster.nl and quitter.nl. The hostname of my server is magistervela.sailmaster.nl.) The quitter.nl mail bounces.