Updated FAQ

FAQ.md

@@ -1069,6 +1069,7 @@ Too large messages and triggering the spam filter of an email server are the mos
 * *554 5.7.1 Service unavailable; Client host xxx.xxx.xxx.xxx blocked*, please [see here](https://docs.gandi.net/en/gandimail/faq/error_types/554_5_7_1_service_unavailable.html)
 * *501 Syntax error - line too long* is often caused by using a long Autocrypt header
 * *503 5.5.0 Recipient already specified* mostly means that an address is being used both as TO and CC address
+* *554 5.7.1 ... not permitted to relay* means that the email server does not recognize the username/email address. Please double check the host name and username/email address in the identity settings.
 
 **Gmail errors**