/usr/share/lemonldap-ng/portal-skins/common/mail_header.tpl is in liblemonldap-ng-portal-perl 1.9.16-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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<body>
<div id="page" style="background:#000;font-family:sans-serif;font-size:12pt;color:#fff;padding:5px 20px;">
<div id="header" style="text-align:right;"><h1>LemonLDAP::NG</h1></div>
<div id="content" style="color:#000;background:#eee;padding:5px 10px;margin:10px 0;">
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