Question about the HOLD header check: why use "Received" ?

[Martijn]

Hello everyone,

While installing and documenting my hopefully 
soon-to-be-fully-operational MailScanner installation, I of course came 
accross the part where you ask Postfix to put everything in the HOLD queue.

I was wondering; why exactly is this done (at least by default) using a 
regex on the Received header?

Is it never so that Postfix receives mail that doesn't contain a 
Received header? I could imagine this to happen for example when a 
website on the same host sends e-mail instead of when e-mail from abroad 
is SMTP'ed to Postfix.

Or, is this not an issue because Postfix adds a Received header before 
it reaches header_check? I have found that header_checks take place in 
the "cleanup" stage ( 
http://www.postfix.org/BUILTIN_FILTER_README.html#what ) which is before 
several other stages ( http://www.postfix.org/OVERVIEW.html ) but I have 
not yet found anything confirming that a Received header is added before 
the cleanup stage is reached.

Just curious :-)

--
Kind regards,
- Martijn