My experience has always been that customers with email addresses at older services (Hotmail, Yahoo, Earthlink, AOL etc) are usually boomers that are using an account set up for them by their millennial aged grandchildren back in the 90's or early 2000's...
This makes perfect sense.
If it's age at all. I'd bet it might be more closely associated with something harder to articulate or measure, along the lines of "a proclivity for clinging to subpar solutions." Someone who uses a subpar email service might also have ingrained poor driving habits.
I can't think of a millennial that would be caught dead with a Hotmail account.
However, I do agree it's age. Probably advanced age rather than young age.
The real problem is that we are allowing "AI analysis" to allow people to discriminate upon something (Hotmail ownership) which we would NOT allow people to discriminate on (age, in this instance) outright.
I guess one worry is that you have multicollinearity, you might get a “double counting” effect on the same signal. But, you should be able to see that in a proper model.
The danger with allowing these kinds of "AI" models is that they will become instances of parallel construction: "We don't want to insure people who are <X>, so construct a model until it coughs up a surrogate to <X> at a 95% probability."