I can’t speak on that front - I’m not experienced enough with SES to know whether this is a problem with our config, or whether it’s a problem with Discourse itself.
I can say that if we want to work as a replacement for mailing lists, proper threading within email clients is a must. So yes, I think we need it.
While I agree, I’ve never seen this problem. I wonder if it’s only an Outlook problem.
Probably, unless we can find a workaround. As I said above, it might be Outlook-only, it wfm on Thunderbird and Postbox. If that’s the case, we’d have to figure out if it’s worth it to replace a known good (other than this) part of our infrastructure. I personally don’t think it is, because this looks like a fringe case.