Actually, there's been a change in the last couple of months with Sassie. I've spoken with my ISP about it and they gave me some information from their side. They believe Sassie has changed some of its coding in some of their outgoing emails, especially in the acceptance ones. I don't receive those from them at all right now if it's a Sassie platform shop. I sent my ISP an 'old' acceptance email from right before I stopped getting them. They said there was nothing in it that would cause it to become spam. He told me if I could get a copy of an acceptance email that is current, he'd see what the cause would be of why it was now being marked spam. Of course, I have no way of getting a copy.
Of note, I recently received an email from an MSC that said Sassie is aware there is an issue and is working on rectifying it. This MSC has taken to sending out their own general notice to their shoppers to tell them they've been accepted for a shop and to go check out their shop logs. So, the increase in spam may have less to do with the various ISPs and more to do with Sassie having changed something in their programming. Having dealt with similar issues in other situations in the past, I tend to think that's the case, especially since I used to get these emails without issue and now they don't even reach the spam folder.
One more thing that may or may not be at issue. My ISP told me that they no longer make that call themselves (spam vs non-spam) but that they utilize a third party specialist that actually does it. It's quite possible that other popular email sources like Yahoo etc that are larger now also utilize some type of third party, if not even this same company, and so correcting anything is extremely difficult because it's not the ISP, it's this third party.