The reason why Im asking about email filtering is because I encountered very sad problem - I have more than 5 mailboxes, every on different hostings.

Some on google, some on yandex, some on my self host, some on other's self host.

And so I need a simple, convenient and reproducible way to move everything to spam filter all of my mail.

I don't want to use proprietary filters - they are, ewww, proprietary.
It is hard to maintain and backup them.

I can't use sieve, because
1) proprietary mailboxes doesn't support it.
2) my personal selfhost too (it can, but i don't want touch dovecot)

I'm also don't like mail client filters for the same reasons as the proprietary.
Despite the backup process easier, maintaining them is like a madness.

Also sometimes i need enogh complex logic in filters.