Ask HN: Email website announcements - expected or spam?

When a user signs up for a web-based service, is it legitimate to send them occasional emails letting them know what's new, or announcing major milestones? Should they explicitly opt-in for this, or are occasional announcements just assumed? I'm not talking about email marketing, an email newsletter, or (of course) any sort of third-party product placement - just an occasional "Hi" from the founder, with a clear "unsubscribe" link.

I want to respect user privacy and definitely don't want to spam or annoy users. But I also want to be able to communicate with people who are using my service.