I've received many company mailings from companies of all sizes over the years and I believe in most cases they are sent by the brokerage firms holding the account, not the company (though I'd bet the company pays the costs and supplies the materials, hence the return address).
I just can't imagine that a public company wouldn't have access to a list of shareholders. As you say, I doubt they get every trade notification though and I'm fairly certain if they could or do, it's not on a real-time basis.
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