Admin directory is included in email copyright link

v1.5.5e
PHP 7.0.10

This is a strange one,

while attempting to understand why my html email formatting is being lost in Windows email programs, I have sent some test emails from within Admin (Send Email to Customers). I've noticed that the copyright hyperlink link in the footer contains the path to my Admin directory i.e.

https://mydomain.co.uk/myadmin/index.php

wierdly my Admin directory has been converted to lowercase in the above link.

in email_template_direct_email.html I have,

 <div class="copyright">$EMAIL_FOOTER_COPYRIGHT</div>

and in includes/languages/english/my_template/email_extras.php I have,

define('EMAIL_FOOTER_COPYRIGHT','Copyright (c) ' . date('Y') . ' <a href="' . zen_href_link(FILENAME_DEFAULT) . '" target="_blank">' . STORE_NAME . '</a>');

and nothing seems awry with my configure.php files.

It may be because I've been at this too long but I assume this can't be right.