Updated FAQ

FAQ.md

@@ -1658,10 +1658,13 @@ This is why texts with dots are sometimes incorrectly recognized as links, which
 Spam filtering, verification of the [DKIM](https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail) signature
 and [SPF](https://en.wikipedia.org/wiki/Sender_Policy_Framework) authorization is a task of email servers, not of an email client.
 
-However, FairEmail will show a small warning flag
+However, FairEmail will show a small red warning flag
 when DKIM, SPF or [DMARC](https://en.wikipedia.org/wiki/DMARC) authentication failed on the receiving server.
 You can enable/disable [authentication verification](https://en.wikipedia.org/wiki/Email_authentication) in the behavior settings.
 
+FairEmail can show a warning flag too when the domain name of the (reply) email address of the sender does not define an MX record pointing to an email server.
+This can be enabled in the receive settings. Be aware that this will slow down synchronization of messages significantly.
+
 If legitimate messages are failing authentication, you should notify the sender because this will result in a high risk of messages ending up in the spam folder.
 Moreover, without proper authentication there is a risk the sender will be impersonated.
 The sender might use [this tool](https://www.mail-tester.com/) to check authentication and other things.