FIX: Specific email error for replies to digest emails (#9770)

lib/email/processor.rb

@@ -66,6 +66,7 @@ module Email
                          when Discourse::InvalidAccess                     then :email_reject_invalid_access
                          when Email::Receiver::OldDestinationError         then :email_reject_old_destination
                          when Email::Receiver::ReplyNotAllowedError        then :email_reject_reply_not_allowed
+                         when Email::Receiver::ReplyToDigestError          then :email_reject_reply_to_digest
                          else                                                   :email_reject_unrecognized_error
       end
 

lib/email/receiver.rb

@@ -32,6 +32,7 @@ module Email
     class UnsubscribeNotAllowed        < ProcessingError; end
     class EmailNotAllowed              < ProcessingError; end
     class OldDestinationError          < ProcessingError; end
+    class ReplyToDigestError           < ProcessingError; end
 
     attr_reader :incoming_email
     attr_reader :raw_email
@@ -190,6 +191,7 @@ module Email
           end
         end
 
+        raise ReplyToDigestError if EmailLog.where(email_type: "digest", message_id: @mail.in_reply_to).exists?
         raise BadDestinationAddress
       end
     end