Document information

Document ID:4690
Subject:Why do I get an error with code 420 when sending emails to Xeams
Creation date:12/14/15 4:35 PM
Last modified on:12/14/15 4:35 PM


Error 420

Problem

You see incoming emails are getting rejected with the following error:

550 5.1.1 No such user here - code 420.

Reason

This error is generated by by Xeams if it detects a dictionary attack. In such cases, Xeams will block the IP address of the sender. A dictionary attack occurs when too many emails containing invalid recipients is sent from one single IP. Consider the following scenario as an example.
  • 200.31.34.8 - an hypothetical IP address on the Internet is trying to send emails to:
    • noreply@yourdomain.com
    • mickey.mouse@yourdomain.com
    • john.doe@yourdomain.com
    • jane.doe@yourdomain.com
    • bill.gates@yourdomain.com
  • Assume all of the email addresses are invalid
  • Xeams will consider this a dictionary attack and will block 200.31.34.8 for the next 10 minutes.
  • The 10 minute counter is always set with respect to the last message - meaning if the first message was sent at 10:00 and the last was sent at 10:05, the IP will remain blocked until 10:20.

Caveats

Although this dictionary attack prevention in Xeams works in most cases, it has a potential of blocking an IP address belonging to one of your internal machines/routers. This happens when another system (either an SMTP server or a router) is sitting in front of Xeams. In this case every incoming message will appear to be coming from the same IP address and therefore, will get blocked.

You have two options if you are running into this issue:
  • Specify a gateway Click here for details.
  • Disable Dictionary Attack Prevention - This is done by following steps:
    • Look for a file called server.properties in $INSTALL_DIR\config folder. If the file is not there, create a new one. On Windows, if you use Notepad, make sure the name of the file is server.properties and NOT server.properties.txt
    • Write the following line in the file:

      dictionary.attack.alerter.enabled=false
    • Save the server.properties
    • Restart Xeams




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