About "per-user mail filter" bugfix:
I'm asked to add a line into /etc/dovecot.conf, but it already exists:

It's /home/vmail by default In iRedMail-0.4.0 and old versions, but is's /var/vmail/ from iRedMail-0.5.0, used to match LSB standard.

And it's clear in post title: Fixed in 0.5.0.

plugin {
    sieve = /home/vmail/sieve/%Ld/%Ln/.dovecot.sieve
}

Just a dubt: is vmail folder set into /var with the last release? I installed iRedMail 0.4 and it automatically set vmail folder into /home.
And I noticed that inside every single path (/home/vmail/sieve/anydomain/anyuser/) there is only a .sieve folder.