My suggestion is to create a filter to intercept and mark it not spam. That's quicker than finding the source. I will share that I also subscribe to the Washington Post and my email provider marks it spam, not Thunderbird. If interested, you might check to see if that what is causing yours to be marked spam.

You need to train the filter in both ways, what's junk, and what is not junk mail. https://support.mozilla.org/kb/thunderbird-and-junk-spam-messages