Python mail filter

[Mailer]

Hello, all,

Well, I need to write a mail filter in Python. The filter will be used in
shared web hosting environment, so the control over MTA configuration etc is
limited.

The basic premise, as I understand is this:

Read mail from stdin
Parse headers etc using rfc822 or email module
Process

# Now I need to do one of the following:

# Discard mail
# Pass through
# Forward to another account, possibly modifying the mail

Now that I have coded up some stuff, the first looks easy - mails are
getting lost. So the question is (may not be entirely specific to Python),
how do I achieve the other two?

Currently, I have set up a .forward that pipes the mail to my script. I can
verify that this works by dumping the contents to a file. If I write to
stdout, however, the mail is not delivered. That doesn't quite look right
either - it's probably too late for the MTA to pick up. What I want to do is
to pass the processed mail back to Postfix so it can deliver it to the
correct local mail box.

FYI, the setup is Sendmail/Postfix.

(P.S. I am very much aware of the existence of procmail, TMDA etc. My
requirements are very specific and requires integration with another
program, so I am only interested in getting a custom solution).

TIA,
Mika