I've seen a shift. Between 2005-2010, I used [company]@[mydomain.com], and I noticed that I would get spam in the form of [gibberish]@[mydomain.com], presumably from spammers who were just targeting email addresses with a catch-call filter. In fact, around that time, my hosting provider, Dreamhost, started restricting email catch-alls to deal with this problem.

But then from around 2010 onward, that type of spam became much less common, and nowadays it's as you say. The vast majority, probably 90%, come from compromised accounts, like linkedin@[mydomain.com]. The rest hit the unique email addresses I have submitted in domain registration forms.

That's even more surprising considering that I've since shifted to using [username]+[company]@[mydomain.com]. Spammers could pretty easily strip off the `+[company]`, but I haven't seen that happen much.