It looks as though it is best practice to include a text/plain version of the message to help with deliverability but I’m not seeing where to enable multipart format. The only documentation for Gravity Forms that mentions multipart is here: https://docs.gravityforms.com/gform_notification/#4-change-the-message-format
Does Gravity Forms allow multipart to be enabled to help with spam filters?
This is not currently a feature of the Gravity Forms notifications, but it is on our list of requested features to add that capability. I will add your vote. Thank you.
add_filter( 'gform_notification', 'change_notification_format', 10, 3 );
function change_notification_format( $notification, $form, $entry ) {
GFCommon::log_debug( 'gform_notification: change_notification_format() running.' );
// Do the thing only for a notification with the name Text Notification
GFCommon::log_debug( 'gform_notification: format changed to multipart.' );
// Change notification format to multipart from the default html
$notification['message_format'] = 'multipart';
return $notification;
}