Good day Dr. B W Darvell, unfortunately there's a complete misunderstanding on your side about the functionality of "Customize headers...". I appreciate the label may be a bit vague, but this is *not* about customizing the list of headers according to your wishes. Like most of Advanced Search Messages, this is about email message headers in a technical sense: Press Ctrl+U on any message and it has many headers like From:, To: , CC:, Subject: and many more which you may have never seen before. For specific advanced uses in the enterprise, users may need to add their own custom headers to the message, like `X-helpdesk-contact: helpdesk@bar.com`. Using certain preferences (see bug 368441), you can handle your "custom" message headers in message reader, advanced search, and composition. You can then compose with such custom headers and set values for them, you can see them in source and in message reader, and you can search for them, and it all works (as seen in screenshot for Advanced Search). It's an advanced feature and if it means nothing to you, it means you don't need it.
Bug 1692183 Comment 5 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Good day Dr. B W Darvell, unfortunately there's a complete misunderstanding on your side about the functionality of "Customize headers...". I appreciate the label may be a bit vague, but this is *not* about customizing the list of headers according to your wishes. Like most of Advanced Search Messages, this is about email message headers in a technical sense: Press Ctrl+U on any message to see the source and it has many headers like From:, To: , CC:, Subject: and many more which you may have never seen before. For specific advanced uses in the enterprise, users may need to add their own custom headers to the message, like `X-helpdesk-contact: helpdesk@bar.com`. Using certain preferences (see bug 368441), you can handle your "custom" message headers in message reader, advanced search, and composition. You can then compose with such custom headers and set values for them, you can see them in source and in message reader, and you can search for them, and it all works (as seen in screenshot for Advanced Search). It's an advanced feature and if it means nothing to you, it means you don't need it.