We don’t have anything set up for that header for emails, so the best I can offer is filtering the following from right at the top of the wp_mail() function.
/**
* Filters the wp_mail() arguments.
*
* @since 2.2.0
*
* @param array $args A compacted array of wp_mail() arguments, including the "to" email,
* subject, message, headers, and attachments values.
*/
$atts = apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) );
If I’m understanding it correctly, a callback for the filter will receive a single array that holds values from each param in the compact() call.
BuddyPress Registration Options only sets a to, subject, and message in our usage, so the headers array index will be empty for the emails we send out, unless something is filtered in by you.
DO note that adding a callback to this would affect ALL emails going out, unless you can work out a way to conditionally set them. I realize that our plugin doesn’t help there at the moment, but we also don’t get many requests for this type of thing, so it just never came up.
