I think you need to walk it through backwards, but maybe one of these?
- Could you have a folder action, Hazel rule, or Keyboard Maestro trigger set to push EML files into the app?
- Check your web services that could link up your e-mail to your system.
- If you think they might come in via a cloud sync’d folder, maybe try deleting the folder and see if you get an error somewhere of a failure.
- Recheck your local e-mail client automations, including plugins.
Another thing you could try is changing your e-mail password and revising any access tokens and seeing if any services complain about not having access, then reconnect them one at a time until your feed starts up again. Similarly you could try disabling your e-mail provider directly in particular web services, but I would expect you would be able to filter for automations using e-mail and whatever cloud file sync services you might use.
Along the same lines, you could turn off any devices you have that could be doing this. A Mac in a cupboard you might run scripts on for example.
Of course, hindsight being 20/20, you have now reached the threshold that you should probably document your automations. I have an Obsidian vault that I keep my documentation about my automations and other solutions. It is very much a retrospective work in progress. I’ve been building it for a few years in various tools as an as-and-when endeavour. As in it is still far from complete.
Anything new goes in, but anything old goes in only when I need to check it, reference it elsewhere, or generally think, I should document that before I forget about it entirely.