Chad White breaks it down for Email Insider.
Question: When is it okay to start emailing people info about company Y, after they signed up for emails from company X?
Answer: It's not.
It doesn't matter that they both have the same parent company, or that it's perfectly legal. It dilutes your list. You lose relevancy and focus. And you create deliverability issues.
Chad highlights good and bad practices -- how to do it properly, and examples of companies you may not want to emulate if you're looking for email success.