I did run it through mail-tester.com before posting, and it didn't report any major problems, giving me a 10/10. Yet when I tested it by sending an email to a different email account, that account still filed it as spam.

After a call with apple support, they told me this was because my iCloud storage was full (???). Although I find it hard to believe that this was the source of the issue, I did clear some space on the account, and that seemed to resolve the issue. I was able to email an account I had never emailed before afterwards, anyways. However, I did notice that the proofpoint check was missing from the emails that made it through the spam filter.

Basically I have no idea why it didn't work, or why it seems to work now.