I just found out I can connect to my SMTP Server via pythons smtplib without the need to login and send mails to whatever address with a random 'from'-address.
I did some relay tests online and they all say it's relay safe.. but... is it a correct behaviour?
As an example:
smpt = smptllib.SMTP('myServer.de')
smpt.sendmail('email@example.com', 'firstname.lastname@example.org', '....;)
This works, and I am doing this from remote, not locally on my Server.