@jwildeboer I don't get it, for Postfix:

Long-term fix

A long-term fix is now available for Postfix 3.8.4, 3.7.9, 3.6.13 and 3.5.23. This stops all forms of the smuggling attacks on recipients at a Postfix server. It introduces a new optional feature that is disabled by default.

--- why is this new security fix disabled by default?