Arguably they're clients as well, since they provide a web-based (client) interface. That aside, I'm wondering how hard it would be to provide these filters in

Arguably they're clients as well, since they provide a web-based (client) interface. That aside, I'm wondering how hard it would be to provide these filters in formats for other clients (I'm thinking of Thunderbird in particular). If nothing else, maybe somebody could convert the GMail/FastMail versions.