Server error 554 Service unavailable Client host blocked using zen.spamhaus.org

Problem Description

You receive an error like: "Server error: '554 5.7.1 Service unavailable; Client host [YOUR-IP] blocked using zen.spamhaus.org"

Problem Resolution

Your mail application is configured to send email without using SMTP Authentication and/or via Port 25. Check your mail account settings in your mail application to ensure:
  1. That it's configured to send (SMTP / Outgoing Server) over port 587 
  2. That the option for SMTP Authentication is enabled, or set to MD5 Challenge-Response or CRAM MD5 -- whichever option you're provided. 
  3. When SMTP authentication is enabled, in some cases the application will prompt for a username and password. Either select the option to use the same credentials as your incoming server or enter your full email address as the username and your email account password.
Be sure to restart your mail application and delete anything from your outbox before attempting to send again, otherwise the messages may continue to attempt to send using the old configuration.

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