@viragomann
Ok, that makes sense. For testing I tried the following:
pfSense SMTP Port: 25
pfSense Enable SMTP over SSL/TLS: off
Postfix on mail server: smtpd_tls_security_level=encrypt (my understanding: this forces the use of STARTTLS)
Error message from pfSense:
Could not send the message to chris@mail.ws3 -- Error: Failed to set sender: root@pfsense.ws3 [SMTP: Invalid response code received from server (code: 530, response: 5.7.0 Must issue a STARTTLS command first)]
--> my conclusion: pfSense does not use STARTTLS
Changing the Postfix setting to smtpd_tls_security_level=may solves the issue but leads to a unsecured connection (not a real problem in my environment, but would be interesting to understand the circumstances)