Overview

Users are getting a Non-Delivery Receipt (NDR) email with the following error messages:

<xxxx@testmail.com> (<server name> 451 Temporary local problem - please try later)

4.1.1 451 Temporarily unable to process your email. Please try again later.

NDR emails may be received when sending a message to an invalid email address.

Diagnosis

When a sender attempts to send an email, the sender’s email client first connects to the sender’s mail server. This mail server connects to the recipient’s mail server via the internet and transmits the email.

The mail, once accepted by the recipient mail server, reaches the recipient’s inbox. The recipient then downloads this mail using his/her email client.

But this email relay process can be affected and mail delivery failures can happen due to temporary issues such as hitting mail limits, DNS errors, unreachable mail servers, etc. This can happen either on the sender’s server or the recipient’s server.

Process

• Check the firewall restrictions.
  Here are some articles that can help you get started with your investigation:
  
  • Configuring Kerio Connect Firewall
  • Securing Kerio Connect
• Check if there are any temporary Local network issues from your office or location, e.g., connection issues, local outages. Double-check the connectivity using tools such as telnet and traceroute to the recipient server from the sender server. Depending on the hop or network where the latency or block is identified, further corrective actions are done to resolve the problem.
• Check if the sender IP address is on the public blacklists. If the sender IP address is on that list, the recipient will reject the mail from that sender. For more information, please refer to Preventing Your Emails from Being Marked as Spam.
• Check the DNS and MX records. For more information, please refer to Working with DNS records.