If you were going to set up a Email Alerts for the customers of your website to subscribe to, what rules of etiquette ought to be followed?

I can think of a few off the top of my head:

• Users can Opt-Out
• Text Only (Or tasteful Remote Images)
• Not sent out more than once a week
• Clients have fine-grained control over what they receive emails about (Only receive what they are interested in)

What other points should I consider?

From a programming standpoint, what is the best method for setting up and running email notifications?

• Should I use an ASP.NET Service? A Windows Service? What are the pitfalls to either?
• How should I log emails that are sent? I don't care if they're received, but I do need to be able to prove that I did or did not send an email.