You would need to create a plugin which overrides qa_send_email(...) and sets $params['html']=true before calling the default function. Your overriding function should be careful to only make this change if it's certain the message is part of a mass mailing. See qa_mailing_send_one(...) for how it's called.
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July 25th, 2023: Q2A 1.8.8 released!