Fix exception for blank body in SMTP server

plugins/messages/app/mail_receivers/messages_mail_receiver.rb

@@ -34,10 +34,7 @@ class MessagesMailReceiver
 
     body.encode!(Encoding::default_internal)
     body = EmailReplyTrimmer.trim(body)
-
-    if body.empty?
-      raise MidiSmtpServer::SmtpdException(nil, 541, "The recipient address rejected your message because of a blank plain body")
-    end
+    raise BlankBodyException if body.empty?
 
     message = @user.send_messages.new body: body,
       group: @message.group,
@@ -75,4 +72,12 @@ class MessagesMailReceiver
     mail_part
   end
 
+  class BlankBodyException < MidiSmtpServer::SmtpdException
+
+    def initialize(msg = nil)
+      super msg, 541, 'The recipient address rejected your message because of a blank plain body'
+    end
+
+  end
+
 end