Updated FAQ

FAQ.md

@@ -1131,6 +1131,9 @@ The matched address will be shown as *via* in the addresses section.
 Note that identities needs to be enabled to be able to be matched
 and that identities of other accounts will not be considered.
 
+Matching will be done only once on receiving a message, so changing the configuration will not change existing messages.
+You could clear local messages by long pressing a folder in the folder list and synchronize the messages again though.
+
 It is possible to configure a [regex](https://en.wikipedia.org/wiki/Regular_expression) in the identity settings
 to match the username of an email address (the part before the @ sign).