Our emails score reasonably well on mail testing tools (see example I just created here). Since February we’ve also created custom sender addresses for every account, rather than binding the reply-to field with a different address (which has become increasingly problematic), we then proxy the replies.
The UUID approach could work, although we’d need to look at the complexity of this vs how widely it would be used. If anyone wants to open a new feature thread for this we’ll certainly look into it.