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mailcow-filter-pdf

This repo lists the steps neccecary in order to activate a sieve filter in mailcow, which filters all emails based on the attachment file extension. A usecase could be forwarding all E-Mails with a .pdf attachment to a separate mailbox for processing by a document management system (e.g. paperless-ngx).

Dovecot config

Go into your mailcow docker folder (where the update.sh script is). Open ./data/conf/dovecot/dovecot.conf with a text-editor of your choice (vim, nano). Look for the sieve_extensions parameter. Add the following options (if not already present): +mime +foreverypart

This is an example how this line could look like:

sieve_extensions = +notify +imapflags +vacation-seconds +editheader

Save and close the file. Go back to your mailcow docker folder (where the update.sh script is). Execute the following to restart the dovecot container:

sudo docker compose restart dovecot-mailcow

Sieve parser

From your mailcow docker folder change to the following directory: ./data/web/inc/lib/sieve

Insert the files mime.xml and foreverypart.xml from this Github repository into that directory. This will extend the mailcow sieve parser (which checks your sieve script before saving) to allow the inclusion of mime and foreverypart extensions in your sieve script.

No restarts are neccecary after this step.

Sieve script

Now open the mailcow dashboard in your browser. Go to E-Mail > configuration > filter and add or open a sieve-filter script for the desired mailbox.

Add the following content and adjust it to suit your needs (replace [email protected]):

require ["mime", "foreverypart", "fileinto", "imap4flags", "mailbox", "copy"];

foreverypart
{
  if header :mime :anychild :param "filename" :matches "Content-Disposition" ["*.pdf"]
  {
    redirect :copy "[email protected]";
  }
}

This script will then tell sieve to send a copy of every mail which contains at least one attached .pdf file to the specified E-Mail-address.

Why shouldn't i check the mime type instead?

The provided script will check the actual filename of the attachement for the presence of a .pdf string at the end. You could also change this script to check the mime type of attachements instead:

if header :mime :anychild :contenttype "Content-Type" "application/pdf"

However, it seems there are a few companies out there (Ebay) which will send attached .pdf files as mime-type application/octet-stream instead. Checking the filename circumvents this issue.