in settings>general there is a tickbox you can untick.

Checking the Settings » General » "Show only display name for people in my address book" check box has the effect of turning on "Prefer display name over message header" for all addresses in the address book.

Unchecking the Settings » General » "Show only display name for people in my address book" check box allows the current setting of "Prefer display name over message header" to apply on a case-by-case basis, which may or may not result in showing the display name from the message header.

What I'm looking for is a way to show the display name from the message header in all cases. It would be nice to be able to turn it fully on or fully off, which probably was the original intent of the Settings » General » "Show only display name for people in my address book" check box. Unfortunately, it only allows one to turn it on. It won't allow you to turn it fully off – temporarily or permanently – for all addresses.

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BTW, there are four formats in which one can export address books. None of those formats include an indication of an address's "Prefer display name over message header" setting. I was hoping to export, run a script on the exported data, then re-import. But since that data isn't included in the export, that's kind of pointless.