Maybe your friend's email server is misconfigured? Incorrect SPF records, etc?

I think the point was that initiating communication with an address is an extremely strong signal of willingness to receive (vs clicking a link, or replying, etc.). I think it's reasonable to expect to receive responses to proactively-initiated threads even if your correspondent isn't optimized for deliverability.

I disagree, just because I’ve emailed [email protected] in the past does not mean I want every spam phishing email pretending to come from [email protected] reaching my inbox.

You make a fair point but you've drawn a slightly broader scenario than I had in mind. Surely if you initiate an email to [email protected] with the title "Hey buddy" and you get back a reply titled "Re: Hey buddy" from someone alleging to be [email protected], you'd want that in your inbox and not spam even with a misconfigured sender on your friend's end... no?

Edit: my ideal UX in this situation would be to get the mail in inbox, with a small notice saying "Unverified" and a mouseover/hover text explaining what that means re: SPF records; from there if you mark it as spam it would treat such unverified mail from that domain as spam on an ongoing basis

yes! I wish I could upvote more.