Updated FAQ

FAQ.md

@@ -714,7 +714,10 @@ and setting the user name field to your main email address.
 Note that you can copy an identity by long pressing it.
 
 Alternatively, you can enable *Allow editing sender address* in the advanced settings of an existing identity to edit the username when composing a new message,
-if your provider allows this.
+if your provider allows this. Considering the email address test@example.org you can use these special username formats:
+
+* Username *+extra* will result in the email address *test+extra@example.org*
+* Username *@extra* will result in the email address *test@extra.example.org*
 
 FairEmail will automatically update the passwords of related identities when you update the password of the associated account or a related identity.