==== Required information ====
- iRedMail version (check /etc/iredmail-release):
- Linux/BSD distribution name and version: Ubuntu 12.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  LDAP
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? Yes
- Related log if you're reporting an issue:
====

Since some days all users suddenly get about half of the spam arriving into their inbox. It is marked as spam (X-Spam-Flag is set to YES, and subject has *** SPAM *** added to it), but it does NOT move automatically to the Junk folder, so the users get to see it in the INBOX.
Curiously, some spam does go into the Junk folder, but some does not even though it's been classified as spam by the server as remarked above.

I do have the standard rule in place in the mail filter sieve, plus an additional one for Amavis:

require ["fileinto"];
# rule:[Move Spam to Junk Folder]
if header :is "X-Spam-Flag" "YES"
{
  fileinto "Junk";
  stop;
}
# rule:[Move content banned by Amavis to Junk Folder]
if header :contains "X-Amavis-Alert" "BANNED"
{
  fileinto "Junk";
  stop;
}

The point is that it has been working for 2 years, and now it suddenly seems to let through about half of it, and still does move the other half.

Any idea what could cause that?