On Fri, 2025-03-21 at 16:56 +1030, Tim via users wrote:

The actual headers you can insert into messags seems to be controlled
from:

/usr/share/glib-2.0/schemas/org.gnome.evolution.plugin.email-custom-header.gschema.xml

<?xml version="1.0" encoding="UTF-8"?>
<schemalist>
  <schema path="/org/gnome/evolution/plugin/email-custom-header/" id="org.gnome.evolution.plugin.email-custom-header" gettext-domain="evolution">
    <key type="as" name="custom-header">
      <default>['Security=Personal;Unclassified;Protected;InConfidence;Secret;Topsecret']</default>
      <summary>List of Custom Headers</summary>
      <description>The key specifies the list of custom headers that you can add to an outgoing message. The format for specifying a Header and Header value is: Name of the custom header followed by “=” and the values separated by “;”</description>
    </key>
  </schema>
</schemalist>

This custom header is a plug-in, and the plug-in manager does let you
edit the options without having to hand-edit the XML file.  But if you
were customising Evolution for a whole organisation, you probably would
want to create custom override XML files.