Problem:
Mails are not delivered
Investigation:
Oct 26 10:49:30 master postfix/smtp[6461]: C53801EC062: to=cc@scheinig.onmicrosoft.com, relay=smtp.strato.de[81.169.145.133]:25, delay=0.16, delays=0.02/0.02/0.11/0.01, dsn=5.7.0, status=bounced (host smtp.strato.de[81.169.145.133] said: 530 5.7.0 User not authenticated (in reply to MAIL FROM command))
Solution:
The authentication file /etc/postfix/smtp_auth was malformed.