Recently, I have been receiving mysterious spam calendar invitations like the one below. You may have received them also.
I never saw a corresponding email. Even worse, the only options are accept or decline. Either one sends a message back to the spammer, confirming my email address! This junk is coming through your iCloud account. Here are instructions for preventing this type of spam from The Dangling Pointer blog.
Since this is associated with your iCloud calendar, it affects both iOS and Macintosh.
They probably didn’t get your email address from anywhere. It appears they are trying big lists of likely email addresses @icloud.com.