Hello all!, @danwaterloo, @duncanc,
After more than a couple of hours of testing, I found a possible bug in PHPList, that kind of messed and rare bug, difficult to locate and reproduce. But if someone can test this in his/her system, and confirm, adding PHPList version and CKEditor version plugin to the report on this thread, that also fails, I will report this as a bug to the PHPList team.
HTML text in a “p” tag thet ends with a link, anchor tag and after link a dot, makes PHPList send the mailing in just plain text because cuts the string in some part of the processing.
Also happens surrounding the a tag with an strong tag, or em tag, or b tag, but with an i tag does not happen.
Example mails to test:
NOT FAILING:
<p>Example text with <a href="https://www.phplist.org">link</a></p>
<p>Example text with <b><a href="https://www.phplist.org">link</a></b></p>
<p>Example text with <strong><a href="https://www.phplist.org">link</a></strong></p>
<p>Example text with <i><a href="https://www.phplist.org">link</a></i></p>
<p>Example text with <em><a href="https://www.phplist.org">link</a></em></p>
<p>Example text with <i><a href="https://www.phplist.org">link</a></i>.</p>
<p>Example text with <b><a href="https://www.phplist.org">link</a></b>.</p>
<p>Example text with <a href="https://www.phplist.org">link</a>. More text.</p>
FAILING:
<p>Example text with <a href="https://www.phplist.org">link</a>.</p>
<p>Example text with <strong><a href="https://www.phplist.org">link</a></strong>.</p>
<p>Example text with <em><a href="https://www.phplist.org">link</a></em>.</p>
TESTED USING:
PHPList 3.2.6 (upgraded from 3.x)
CKEditor plugin version 2.1.3+20160603 by Duncan Cameron
Linux system with Apache and MySQL
STEPS TO REPRODUCE THE BUG:
- Create a new campaign.
- Compose message using HTML and CKEditor.
- Using HTML/source view, paste one of the “Example mail
elements”
- Send a test mail to someone that supports HTML format.
Hope this helps someone, and free from some headaches 
Iñaki López
ignacio.lopez@paginaweb4u.com