Hello @ChetD2 ,
Here are some general best practices for deliverability.
Deliverability can have a lot of variables, from elements on our side, on your side, on the recipient's side, and in their email system company's side. It's also a generally good idea to set up additional authentication for your emails - whether that's through us if you have a free domain (e.g. Gmail, Yahoo, AOL, etc.) or via self-authentication if you have your own domain (such as through a business or other organization).
If you'd like to learn more about safelisting, and what it entails:
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Deliverability agents.
In regards to your specific question about SpamAssassin, if it's still affecting your emails even after you're no longer sending through us then it sounds like something in your emails is causing it to trigger. The content, the amount of images vs text vs links, any glaring typos, or even potentially the sending reputation of your domain / sending address can all be potentially contributing to this. However, since we're not SpamAssassin, we don't really have any insight or troubleshooting we can offer in regards to their specific system or its functionalities. I'd recommend reaching out to their support for that.