If you're sending emails that most recipients aren't interested in, don't you have bigger problems than whether they had to waste ten seconds opening the email before they noticed its worthless?
> Instead of forcing your way into their inbox, you’re confirming and affirming an action they took—and you can always add a dose of cross-sell/up-sell into those messages.
Be very careful with that. It could be an easy way to make users hate you. If I'm on your site, I don't need to get an e-mail for every other click I make. I've already established a communication channel - namely, your website. It would be like you were texting stuff to me during a face-to-face conversation between us.
> But if the primary message is related to a user action, users will be less likely to drag that message over to the promotions tab.
If the message is about something you could have just displayed to me when I was performing that action, then be damn well sure I'll skip the "promotions" tab. I'll drag it straight to the spam. On principle.
Make sure the user wants emails from you. Don't be a dick and check the checkbox to 3-4 mailing lists on sign-up. Maybe add a link when they confirm email to opt into messaging.
The present default to on for most mailing lists on sites is a smell. Fix that first.