It’s usually the other way around, where a sender’s ( not a receiver’s ) email gets blacklisted. If you’re getting the notification emails, but he isn’t, then it sounds like Hotmail may be blocking Webflow’s messages.
He can try whitelisting it, most mail providers have a way to declare that certain domains / emails are legit and should be allowed through.
Alternately, skip Webflow’s forms handler and use Basin or Formspark, a different form submission handler can cut out spam and give you full visibility into what’s been captured ( spam included ). I find that much safer than a black-hole spam rejection approach. This is my client’s business, so I need assurances that sales enquiries are not getting tossed with the spam.