Updated FAQ

FAQ.md

@@ -2867,7 +2867,12 @@ Since this is an experimental feature, my advice is to start with just one folde
 
 *Send address unknown (version 1.1477+)*
 
-Send a [Delivery Status Notification](https://tools.ietf.org/html/rfc3464) *Address unknown* via the reply/answer menu.
+Send a [Delivery Status Notification](https://tools.ietf.org/html/rfc3464) *Address unknown* (=hard bounce) via the reply/answer menu.
+
+Hard bounces will mostly be processed automatically because they affect the reputation of the email provider.
+The bounce address (=*Return-Path* header) is mostly very specific, so the email server can determine the sending account.
+
+For some background, see for [this Wikipedia article](https://en.wikipedia.org/wiki/Bounce_message).
 
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