Errado. Existe a filtragem header_checks, confiram:
FILTERS WHILE RECEIVING MAIL
Postfix implements the following four built-in content inspection
classes while receiving mail:
--> header_checks (default: empty)
These are applied to initial message headers (except for the
headers that are processed with mime_header_checks).
--> mime_header_checks (default: $header_checks)
These are applied to MIME related message headers only.
This feature is available in Postfix 2.0 and later.
--> nested_header_checks (default: $header_checks)
These are applied to message headers of attached email messages
(except for the headers that are processed with
mime_header_checks).
This feature is available in Postfix 2.0 and later.
--> body_checks
These are applied to all other content, including multi-part
message boundaries.
With Postfix versions before 2.0, all content after the initial
message headers is treated as body content.