But I certainly do use metadata about people's communication in my general assessment of their relevance, trustfulness, clue level, etc. Do you use a Hotmail address? Well then, you really are out of it. Gmail? A lot sleaker, yes, but our conversation will be under third party scrutiny, and chances are that you haven't thought that part of it through. Your work mail for a private correspondance? I wouldn't do that in a million years, so yeah, your total score just went down a notch. Is that an Outlook client I see you're using? Fine, but sort of humdrum, and you are probably the kind of person who will send me .docx documents to read, and make a fuss when you get them back, formatting screwed up. And so on and so forth, most of it subconscious, but the evaluations stick, and mostly turn out to be accurate.
And yes, I have made first sortings of job applicants on the the same kind of criteria.
I dare say it is more powerful than gmail and just as modern. In fact, much more modern; gmail hasn’t changed much in the last 5 years (or longer).
I have an outlook.com address primarily, but it’s tied to the same hotmail address that I used back in 2006. Yes, Hotmail gives you multiple email addresses for free, and gives you all sorts of options for managing them (and yes, they work for sign in).
It's not a farfetched idea that the person sending a job application from the web client of a Gmail account is probably a different type than the one with a personal domain and running Thunderbird/Linux.