That's a shame, really, because Outlook (and Outlook Express) was just so terrible at managing email. I got my parents switched off of that years ago though, so haven't looked at Thunderbird since.
These days nearly everyone uses web mail. Whether it is Zimbra or OWA, the quality of webmail has gotten so good that rarely do you want to do anything else except on your mobile devices. And if you want to go offline, Exchange uses to expect to go to Outlook and Zimbra has its own client too. The number of "working offline on non-mobile devices IMAP users" are dwindling. Thunderbird and Evolution cover that gap pretty well.
But perhaps I should add a not in the sign up agreement that "by signing up you agree to receive emails from us" or something like that, to be safe.
If you're in the EU, this wouldn't constitute as opt-in. The EU states that a user must specifically click a button that states "I want to receive these emails". Unless it's regarding their account. So, if the daily digest included notifications, it would be fine, but if it was just new topics, I'm not so sure. It should be fine either way, as you've said.