email: Implement spam sorting with sieve

- Sieve script detects spam headers used by Rspamd and SpamAssassin
- Autosubscribe to Archive, Drafts, Junk, Sent, Trash
- Autoexpunge Trash and Junk folders
- INBOX.Junk -> Junk, INBOX.Trash -> Trash, Sent Messages -> Sent

plinth/modules/email_server/audit/spam.py

@@ -1,7 +1,9 @@
 """Configures spam filters and the virus scanner"""
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
+import glob
 import logging
+import subprocess
 
 from plinth import actions
 
@@ -50,3 +52,21 @@ def fix_filter(diagnosis):
 def action_set_filter():
     with postconf.mutex.lock_all():
         fix_filter(check_filter())
+    _compile_sieve()
+
+
+def _compile_sieve():
+    sieve_list = glob.glob('/etc/dovecot/freedombox-sieve-after/*.sieve')
+    for sieve_file in sieve_list:
+        _run_sievec(sieve_file)
+
+
+def _run_sievec(sieve_file):
+    logger.info('Compiling sieve script %s', sieve_file)
+    args = ['sievec', '--', sieve_file]
+    completed = subprocess.run(args, capture_output=True)
+    if completed.returncode != 0:
+        logger.critical('Subprocess returned %d', completed.returncode)
+        logger.critical('Stdout: %r', completed.stdout)
+        logger.critical('Stderr: %r', completed.stderr)
+        raise OSError('Sieve compilation failed: ' + sieve_file)

plinth/modules/email_server/data/etc/dovecot/conf.d/90-freedombox-lmtp.conf

@@ -0,0 +1,6 @@
+# Direct edits to this file will be lost!
+# Manage your settings on Plinth <https://localhost/plinth/apps/email_server>
+
+protocol lmtp {
+  mail_plugins = $mail_plugins sieve
+}

plinth/modules/email_server/data/etc/dovecot/conf.d/90-freedombox-mailboxes.conf

@@ -0,0 +1,43 @@
+# Direct edits to this file will be lost!
+# Manage your settings on Plinth <https://localhost/plinth/apps/email_server>
+
+namespace inbox {
+  mailbox Archive {
+    auto = subscribe
+    special_use = \Archive
+  }
+  mailbox Drafts {
+    auto = subscribe
+    special_use = \Drafts
+  }
+  mailbox Junk {
+    auto = subscribe
+    autoexpunge = 30d
+    special_use = \Junk
+  }
+  mailbox Trash {
+    auto = subscribe
+    autoexpunge = 30d
+    special_use = \Trash
+  }
+
+  mailbox INBOX.Junk {
+    auto = no
+    autoexpunge = 30d
+    special_use = \Junk
+  }
+  mailbox INBOX.Trash {
+    auto = no
+    autoexpunge = 30d
+    special_use = \Trash
+  }
+
+  mailbox Sent {
+    auto = subscribe
+    special_use = \Sent
+  }
+  mailbox "Sent Messages" {
+    auto = no
+    special_use = \Sent
+  }
+}

plinth/modules/email_server/data/etc/dovecot/conf.d/95-freedombox-sieve.conf

@@ -0,0 +1,6 @@
+# Direct edits to this file will be lost!
+# Manage your settings on Plinth <https://localhost/plinth/apps/email_server>
+
+plugin {
+  sieve_after = /etc/dovecot/freedombox-sieve-after
+}

plinth/modules/email_server/data/etc/dovecot/freedombox-sieve-after/sort-spam.sieve

@@ -0,0 +1,8 @@
+require ["fileinto", "mailbox"];
+
+if anyof(header :contains "X-Spam" "Yes",
+         header :contains "X-Spam-Status" "Yes",
+         header :matches "X-Spam-Flag" "YES") {
+  fileinto :create "Junk";
+  stop;
+}