Or just a create an e-mail account which is connected to your small server with IMAP with IDLE.

Create a white list, and if mail is coming from a white-listed mail then print the contents, otherwise discard the mail.

This is how my Deskjet 4515 is working besides GCP. It's a pretty neat solution. It even replies with a result e-mail. I hope that HP won't kill its services too. They've already killed the app side, but remote printing is still intact.

Probably easy enough to throw this together in python and host it on your choice of cheap, lower power hardware (Raspberry pi, etc).

It'd also be very trivial to script with a Mac using Folder Actions and a 5 line AppleScript.

(I wish Linux had an equivalent to Folder Actions. They’re really nifty.)

The Linux equivalent of Folder Actions would probably either be inotify¹ or inotify-tools².

1:https://linux.die.net/man/7/inotify

2:https://github.com/rvoicilas/inotify-tools/wiki

A much easier to use tool that works using inotify is incron. It's basically like cron except for inotify events. I have a one-liner that OCRs all of my incoming scans, and it's immensely helpful.

What do you use to do the actual ocr?

A python package called OCRmyPDF. It has same defaults and works very well for my use case.