You can’t forward e-mail from a mailing list into Discourse. The potential for mail loops and other forms of rampant insanity is just too high, and the legitimate use cases are too few to justify playing whack-a-mole with the stupendous number of corner cases that need to be handled.
The only idea that comes to mind would be to get Yahoo to forward the e-mails to some sort of intermediate that would strip out the problematic headers before sending them on to Discourse, but… eeeeew. I’d recommend rethinking the plan that has you forwarding the e-mails from Yahoo to Discourse in the first place.