Allow to use Sieve filters

[clochix]

Sieve is a popular way to filter emails on server side. It's both

• a language to describe filters and actions;
• a protocol to manage filters on a Sieve enabled Email server.

We can't assume the remote server allows to manage filters, so we should probably allow the user to upload some Sieve scripts into the Email application, and apply the filters on our side.

A lot of tools exist that allow to play with Sieve. Unfortunately:

• lots of them are bundled with servers;
• it's hard to distinguish between library that only parse Sieve scripts, library to manage remote filters, and library that can really filter a message given a script;
• there don't seem to be any JavaScript binding.

I had a look at http://sieve.info/libraries but didn't find something we could reuse to easily implement Sieve filters inside CozyMails. GNU/Mailutils offers a command-line tool that allow to filter emails, but it seems to only be able to operate on local mailboxes, not thru IMAP.

If we want to use Sieve, I thing the best choice would be to use the libsieve library, available in Debian. It's source code embed some tests using a command line client. We can use this sample program, running it on every message and parsing the output. Or we can get some inspiration from the test program to write a nodejs wrapper (but it won't be easy). See https://packages.debian.org/source/sid/libsieve (Version shipped with Debian is pretty old but I'm unable to compile the latest from https://github.com/sodabrew/libsieve)

See also:

• http://sieve.info/
• https://en.wikipedia.org/wiki/Sieve_%28mail_filtering_language%29

[m4dz]

Maybe we can implement those filtering capabilities in to complementaries way:

1. If the IMAP server supports SIEVE filtering (available behind a dedicated sieve port), then filtering is executed remotely, and CozyMail just pass scripts to IMAP server through ManageSieve protocol
2. If the IMAP server does not, so filtering is executed locally inside CozyMail, using the SIEVE scripts or not.

This way, we can implement filtering in two times, taking care of IMAP capabilities. We also can have a single unified interface to editing filtering rules that generates scripts for ManageSieve, or just apply filtering locally using the internal selected protocol. For users, it's transparent and do not rely on a technical background (aka "I don't want to editing sieve filters myself, it's too boring"). Also, we can provide an advanced view we users can simply copy/paste their already designed sieve scripts that can be read and used by CozyMai.

My 2cts :wink:…