Overview

While trying to send emails to a specific address, Kerio Connect generates the following Mail Delivery Subsystem notification:

Result: failed, Status: 5.4.4 Host not found, Remote-Host: 127.0.0.1

The issue appears to be happening only with a specific email address.

Diagnosis

The error message indicates there is a DNS issue with the reported host. Usually, it indicates the DNS records misconfiguration or network/firewall restriction. It might be an issue with the email address itself, such as the mailbox is not presented in the mail server domain configuration or it might be a small typo in the mailbox.

The mailbox validation can be done using external tools like VerifyEmailAddress. If the domain does not exist, the website may return such information:

An alternative way is to check MX records configured for the necessary domain in MXtoolbox. Sometimes, the full domain name is entered incorrectly in the compose email section, i.e. domain.3a.fr instead of domain3a.fr

The correct domain returns the corresponding MX records info.

Solution

Double-check the recipients' email address to make sure there are no typos in username or domain name. Reach out to the recipient using alternative mailbox or other methods (call, meeting, chat, etc).

Confirmation

The email is sent successfully with zero NDRs (Non-Delivery receipts).