
Looks like a LOWESS curve (https://en.wikipedia.org/wiki/Local_regression). They always overfit, but are still useful to show trends. The main danger is that they get wonky near the limits of the data. Note that that increase at the end of the left plot for Democrats looks like it is increasing -but that increase looks to be 100% dependent on that single data point for '25. Obviously, you never want your analysis to be dependent on a single point like that.
I use sendgrid as my outgoing smtp relay to avoid ip reputation issues you mention. You still have to configure your dns settings for spf and dkim pointing at their servers instead of yours. Their free tier is 10x the email I’ll ever send so it doesn’t cost anything. There are a few companies in this space with free tiers. It works, but it isnt Gmail level deliverability. I still get spam binned occasionally.