[Dovecot] Add sieve rule to move DeltaChat (https://delta.chat) messages to folder DeltaChat

[Web] Add information about extended DNS config

data/conf/dovecot/global_sieve_before

@@ -1,2 +1,14 @@
 # global_sieve_before script
 # global_sieve_before -> user sieve_before (mailcow UI) -> user sieve_after (mailcow UI) -> global_sieve_after
+
+require "fileinto";
+require "mailbox";
+
+if header :contains ["Chat-Version"] [""] {
+  if mailboxexists "DeltaChat" {
+    fileinto "DeltaChat";
+  } else {
+    fileinto :create "DeltaChat";
+  }
+  stop;
+}