I do the same thing, but more like company@mypersonaldomain.com. I don't think that most companies sell or give away my email addresses, but they give lists to their MARKETING PARTNERS, which certainly do pass them on, or get hacked. I found this out by checking the to lines in spam and seeing united (airlines) and a CMS vendor. I also saw something from a company mailed to the email address associated with one of its competitors; from talking to people I found that a marketing person left the second and apparently took a contact list to the their new job at the first. It is a very worthwhile thing to do.