Use case is private email support portal. We have a private category that a handful of users can access. The general public can send emails to firstname.lastname@example.org and they are posted in our public emails category. When one of the discourse users replies, it gets posted in the topic, and the member of the public (now a staged user) gets the reply in an email. This all works well.
As of about a month ago (I just noticed this now and don’t know exactly when it started) when that staged user replies to the topic they started their email is rejected:
Email can not be processed: Something has gone wrong. Perhaps this topic was closed or deleted while you were looking at it?
This is happening even though the topic is certainly not closed or deleted. Recently updated to 1.6.0.beta8 and the problem persists.