Add note about case conversion and how to fix it

docs/u_e-mailcow_ui-tagging.md

@@ -1,5 +1,4 @@
 Mailbox users can tag their mail address like in `me+facebook@example.org`. They can control the tag handling in the users **mailcow UI** panel.
-
 ![mailcow mail tagging settings](images/mailcow-tagging.png)
 
 ### Available Actions
@@ -7,3 +6,20 @@ Mailbox users can tag their mail address like in `me+facebook@example.org`. They
 1\. Move this message to a sub folder "facebook" (will be created lower case if not existing)
 
 2\. Prepend the tag to the subject: "[facebook] Subject"
+
+Please note: Uppercase tags are converted to lowercase except for the first letter. If you want to keep the tag as it is, please apply the following diff and restart mailcow:
+```
+diff --git a/data/conf/dovecot/global_sieve_after b/data/conf/dovecot/global_sieve_after
+index e047136e..933c4137 100644
+--- a/data/conf/dovecot/global_sieve_after
++++ b/data/conf/dovecot/global_sieve_after
+@@ -15,7 +15,7 @@ if allof (
+   envelope :detail :matches "to" "*",
+   header :contains "X-Moo-Tag" "YES"
+   ) {
+-  set :lower :upperfirst "tag" "${1}";
++  set "tag" "${1}";
+   if mailboxexists "INBOX/${1}" {
+     fileinto "INBOX/${1}";
+   } else {
+```