Is there a configuration flag that I can use to set ignore_8bit_checks to true?
Ok, that was dumb. It’s here: ignore_8bit_checks - KumoMTA Docs
FWIW, ignore_8bit_checks = true should be considered to be a last resort, as 8-bit messages are not guaranteed to be relayable via SMTP. While turning off that check may get you to the next hop in this specific circumstance, it is not a good blanket default.
That’s why the documentation for that option and the error message that we generate are pushing you towards fixing the content rather than explicitly ignoring what the SMTP protocol tells us we should be doing.