Sorry for the long wait on this. You have it right, the rejections are coming from ImprovMX, which receives mail for newsletters.newsblur.com, and their SpamAssassin scoring bounces anything at 5.0 or above before NewsBlur sees it. NewsBlur itself doesn’t filter newsletters at all, and I agree the secret token in the address should be enough. I’ve asked ImprovMX to turn off filtering for our domain since the mail only goes to NewsBlur and never to a mailbox. If they won’t, the next step is moving newsletter delivery off ImprovMX entirely. I’ll post here when I know which.
This started happening on other newsletters so I finally took a look: WIRED Newsletters blocked as spam