This may go without saying, but there is a way to customize that message by digging into the Discourse code. However that poses two issues (possibly):
Will the changes persist through Discourse version updates?
Not everyone has the wherewithal to dig in and modify code (especially on their production instance).
Lauren is looking for a way for an average Discourse administrator to be able to modify this email template. I imagine it belongs in a bigger discussion of allowing customization of all content templates, but for now, we can just stick to the rejection email template.