They say that the problem is likely due to a new outgoing spam filter they've installed

You realize what you wrote, do you? A new outgoing spam filter they've installed.

any solution would need to take place on Thunderbird's end.

I cannot follow that logic.

Thanks. I should have mentioned that outgoing email sends fine from the Bluehost platform on a web browser. So it seems to have something to do with the interaction between the Bluehost outgoing spam filter and Thunderbird, rather than the outgoing spam filter itself.

My first response to Bluehost support was also "hey, it seems like if y'all introduced an outgoing spam filter that blocks all legitimate outgoing mail from a popular third party client, that's y'alls problem, not Thunderbird's." But they disagree, so the next step would seem to be to figure out if it can be resolved on Thunderbird's end.

They know what they have put in place. Why don't they say what they think needs to be done?

You realize what you wrote, do you? A new outgoing spam filter they've installed.

any solution would need to take place on Thunderbird's end.

I cannot follow that logic.