Our upstream MTA is being temporarily disabled but responds with "550 contact support". This means all our automatically generated emails are going down the drain.
How do I configure Postfix so that the hard bounce is ignored and the mails deferred?
soft_bounce = "yes"
does not help, because it only affects my own responses.
You're confusing a few things.
When an upstream SMTP server sends you a 550 response, that means the upstream server is rejecting your mail; it is not "being bounced".
Your server would be the one generating the bounce message in response to that 550.
soft_bounce = yes
does in fact directly influence the above, and setting it means that your server reacts to every 5xx response with a defer action for later retry (up to the configured limit).From the documentation:
If you think this is not happening, include some logs that show this.