Is it possible to use multiple flags on a single email in Mac Mail

I'm still using El Capitan, for a number of reasons.

So in Mac Mail, I have a choice of six different colored flags. I want to be able to flag emails more than once – that is, with multiple colors.

Actual use case – I run an online literary magazine, and I'm flagging emails as Orange (to read), Green (accepted) and Red (rejected). BUT I would love to be able to add a Gray tag to accepted and rejected emails (eg, Green and Gray – accepted and published, Red and Gray – rejected and recipient notified). Anyway, there are multiple cases where this would be useful.

Is it possible?

Strangely enough, when I've sent emails back and forth to other folks, I have seen multiple flags! But they're not in the usual spot (left side of user's email, top email) but to the right of the user's email (bottom email, green and gray!) I have no idea how this happened and have not been able to replicate it, but it's exactly what I want.

Thanks!