Allow filenames based on headers

[Alex Martin]

Hello,

We have recently started to block incoming emails with a zip file
attachment.

Here's the bit from our amavisd.conf file:

%banned_rules = (
  'BLOCK_ALL' =>  new_RE(
    [ qr'^text/plain$'i => 0 ],
    [ qr'^\.[a-zA-Z0-9]*$'   => 1 ] ),

  'BLOCK_ZIP'   =>      new_RE(
    [ qr'^\.zip$' ],
    ),

  'DEFAULT' => $banned_filename_re,
);

@banned_filename_maps = ({
  '.domain1.co.uk'       =>      'BLOCK_ZIP',
  '.domain2.com'         =>      'BLOCK_ZIP',
  '.domain3.net'         =>      'BLOCK_ZIP',
  '.domain4.co.uk'       =>      'BLOCK_ZIP',
  '.domain5.co.uk'       =>      'BLOCK_ZIP',
  '.'                    =>      'DEFAULT',
});

However, naturally now we need to make exceptions. I realise that we can
set the above based on recipients' email addresses. But ideally I would
like to know if we can make exceptions based on headers (as sender can be
easily spoofed).

Something like if email appears to be from smtp.example.com allow zip file
attachments.

Does anyone know if this is possible or have any recommendations we can try?

Many thanks,
Alex
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