How can I stop notifications from being marked "Maybe Spam" in Messages?

I accidentally marked a text alert from Venmo as spam when I deleted it in Messages. Now all messages from Venmo are marked as "Maybe Spam" and do not pop up in my desktop alerts. This affects sign-in as well since the code Venmo sends is also marked "Maybe Spam." How do I un-flag this number as spam? I have tried undeleting the messages and then deleting them again, but nothing has changed. I don't see a way to remove this flag anywhere.