That's a great idea. I do the same thing as the parent poster, and I've seen lots of email addresses compromised. If it's one that I care about, I will contact the company. (Last time it was Tektronix, before that it was Roku.) They usually deny that it could have been on their end, which is BS because nobody besides them ever had the email address (assuming I wasn't hacked locally). The most common scenario is the insider threat. Some low-paid IT person with access to their database sells the list for a few tens of dollars.

Obligatory mention: https://haveibeenpwned.com