I would argue the OP is correct gmail's implementation of labels is a proprietary extension of the IMAP standard. Now you are correct that it's not so relevant

I would argue the OP is correct gmail's implementation of labels is a proprietary extension of the IMAP standard. Now you are correct that it's not so relevant when exporting to mbox. It should also be noted that placing labels into the message headers is a solution that can lead to information leaks, and one should be careful with it The headers are kept (at least in some email apps) when you reply to those emails so the receiver will know your label for that message. So if you label email from your boss as "from idiot" he will know (and there are obviously more serious examples). Regarding the Gmail labels in IMAP, I'm not sure why you talk about headers. The page you linked to is about the IMAP extension commands, AFAIK the labels do not end up in the headers in that case.