Ask HN: SaaS apps and email lists - double opt-in? opt-in? required?

I'm curious how folks running SaaS apps are handling email. I'd like to set up an autoresponder for my trial members and maybe even my paying customers. But how to get them on the list?

Do you force them to opt-in to receiving emails as a term and condition of the site? Do you ask them to opt-in using a checkbox when they're registering/activating their account? Do you ask them to double opt-in by using a checkbox on the site, mailing them and having them confirm by clicking the link.

For my trial users, I'm thinking simple things: * thanks for signing up, here's a guide to help you get started * 1/2 way through – how's the trial going, anything I can do to help * -5 days – your trial is almost up, don't forget to fill out your CC details to keep using the service.

For existing users * blog posts * new features * new whatever (questions in the FAQ, videos in help, etc, etc) * limited to 1-2 times a month, max.

Appreciate any feedback/links/personal experiences running or using SaaS in relation to email marketing.