Owkay, we solved it!!
After setting in '/etc/postfix/master.cf' the -D option after our SMTP rule. Then looking to our log file I noticed the error:
logger: open("/usr/lib64/sasl2/smptd.conf", O_RDONLY) = -1 ENOENT (No such file or directory)
Conclusion, we had a typo in out '/etc/postfix/main.cf'
smtpd_sasl_path = smptd
In place of:
smtpd_sasl_path = smtpd
I know it's stupid, but when you already configuring and tweaking your mail server for days such typo's are so easy to overlook..
Thx Falco for noticing us about the smtpd.conf path!