I've checked with another recipient, who found my message to her Hotmail address in her Junk folder. On learning this, the first friend I checked with also checked his "Junk" folder (apparently not having previously realized that this was distinct from his "Unwanted mail" folder), and found my messages there, too.
So the next question is this: Why is Hotmail putting ordinary friendly emails from my domain into people's Junk folders? My IP address obviously hasn't been sending large volumes of unwanted mail, because senderscore.org still reports "Insufficient Mail Seen" from my IP address, and therefore won't give me a Sender Score. Also, mxtoolbox.com reports that I'm not on any blacklists.
I tried opening a ticket with Microsoft, but the result was that my IP address is "Not qualified for mitigation", whatever that means. I tried asking why it wasn't qualified for mitigation, but Microsoft seems determined not to give me any useful information, saying only that "I do not see anything offhand with the IP ... that would be preventing your mail from reaching our customers", and giving links to very generic information, much of which seems relevant only to businesses with mailing lists, not individuals like me running their own mail servers.
When I was first searching the internet for other reports of this problem, I came across { URL that doesn't state that - redacted by moderator}, which says "Hotmail won’t allow companies to email you unless they pay for the privilege (yes, they want people to pay money to send Hotmail members email)". At first I was sceptical, but one of the suggestions Microsoft gave me was to apply for the Sender Score Certified Mail programme, which I understand costs money. So it's starting to look a little more plausible now.