>I've just started using IMGate, and so far am very pleased.  I've read the
>manuals, yes read, yes all of them :)  some of the more relevant ones
>twice.  What I still don't understand is RegExp.  I'm trying to figure out
>exactly what I'm denying in the header_checks.regexp file (default basic
>install).

headers are SMTP-defined (some official, some not) and are part of the DATA 
command. lines like:

Received:
X-Persona:
Delivered-To:
X-Sender:
X-Mailer:
Date:
To:
From:
Subject:
Mime-Version:
Content-type:
Content-Transfer-Encoding:
X-listar-version:
Sender:
Errors-To:
X-original-sender:
Precedence:
Reply-To:
List-help:
List-unsubscribe:
List-software:
X-List-ID:
List-subscribe:
List-owner:
List-post:
List-archive:
X-list:
X-RCPT-TO:
X-UIDL:
Status:
Content-Type:

They are not part of the "SMTP envelope" info. That info is part of the 
other, non-DATA SMTP commands and the TCP connection:

connect from  MTA  unknonwn[ip.ad.re.ss]

or

connect from  MTA  PTR_domain[ip.ad.re.ss]

then:

HELO / EHLO hostname

MAIL FROM:<[EMAIL PROTECTED]>

RCPT TO:<[EMAIL PROTECTED]

>If someone knows where I can find _good_ information on RegExp

O'Reilly has a book of RegExp, including comparisons of POSIX vs PCRE vs 
whateevr regexp, but it's pretty heavy going, as it means to be the 
reference treatment of regex in all its parsing, mathematical glory.

"Learn PERL" books have often sufficient-for-mail-filtering regex 
introductions.  and just Google around.

Len


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