I don't think there's actually a conspiracy to block the political email messages of any certain party. I see no clear data suggesting political bias in spam filtering decisions. So where did this idea come from? Representative Greg Steube (R-Florida) complained to Google's CEO last week that his campaign emails going to the Gmail spam folder and how that this is supposedly happening only to Republicans. Groan. Not true. Mail is filtered based on whether or not the mechanisms the ISPs have in place can identify it as wanted or unwanted. It's that simple. Yes, those filters are complex. Yes, they are possibly even imperfect. No, they are not out to get you.
The real issue is that, "by and large, political senders are simply not great senders," and Nicky Copland from Validity explains why. Spam is spam. Spam is unwanted, and ISP spam filters try hard to keep unwanted mail out of view. And political senders aren't exempt.
In spite of what Rep. Steube claims, the data collected by the Markup back in March shows that political senders from both parties can end up in the spam folder. Affected Democrats included Beto O'Rourke, Kamala Harris and Andrew Yang. And let's not forget Tulsi Gabbard's lawsuit against Google last year. It has since been dismissed, but one of the suit's allegations was that Gabbard's emails were being placed in spam folders on Gmail at “a disproportionately high rate” when compared with emails from other Democratic candidates.
In short, Democrats get blocked, too. And like Nicky Copland says, any blocking, affecting someone affiliated with either main party, is very likely due to send practices, not due to their political viewpoint. If I were to suspect some commonality in a group of senders all going to the spam folder, I know from experience that the common thread is probably similar (poor) sending practices. It could in theory be due to something else, but there's no compelling data suggesting so.
The Markup missed the boat in their coverage, seemingly not touching on or showing an understanding of what email reputation is and how spam filters work. And their dataset was limited -- it covered only 2020 presidential candidates, but I think it's enough data to show the (lack of a) conspiracy.
And this kind of misunderstanding of how spam filters work -- looking for a conspiracy where none is to be found, is hardly new. Remember DearAOL.com?