Actually, I had just resolved the issue on our side with a serious, unrecommended hack. I set the $header variable to ''. Now it works exactly as I want it to, since we do not have normal e-mailing except in one very special case.
Everything else in our setup is routed through the internal messaging system. This keeps students from e-mail each other and keeps everything onsite.
Resource types
ERROR: Could not instantiate mail function
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