SMTP 550 A URL in this email ( companies . to ) is listed on

My wife recently reported a problem that emails sent from Thunderbird were being rejected. Emails to certain email addresses, including my own, were being returned as undeliverable. She asked me to look into it.

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
host []
SMTP error from remote mail server after end of data:
550 A URL in this email (companies . to) is listed on Please resolve and retry

I asked Scott McKeown of Red IT hosting for ideas. He replied

Does your wife’s email have a signature block in it that includes a URL?

I replied in the affirmative, removed the email signature, and the emails starting going.

Problem Analysis

So then we looked at the email signature she was using. It contained her contact details: Name, Company, Web address, email, phone numbers and social media links for Twitter, Facebook and LinkedIn.

I’d created it for her many years ago using my oik-email-signature plugin, which creates the required HTML using data from your website. The input mostly consisted of shortcodes.

[bw_contact] - [bw_company]
<a href=""> [bw_domain]</a> 

The plugin expands the shortcodes to produce raw HTML. This HTML output had been copied and pasted into Thunderbird’s Account Settings Signature text field, with Use HTML checked. Inspection of the HTML soon revealed the culprit URL in the Follow me link to her Facebook page.

<a href=""
title="Follow me on Facebook">
<img class=""
title="Facebook" alt="Facebook" />

We changed the URL to the current setting and reinstated her Email signature.


Many years ago was a valid shortening of the Facebook URL for business pages. Here’s a sample screen capture from my documentation for the plugin, dated 30th December 2012.

Screenshot for oik-email-signature showing link to Facebook

We don’t know when it changed; it’s no longer owned by Facebook. Nor how recently the domain name was added to’s blacklist.


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