Where I work, we send e-mails to our customers when they checkmark "Send me e-mail about this..." on our web-sites.

We have quite a few web-sites & customers, and a lot of people want us to send them e-mail about our products etc.

Our system admin told me that we got marked as spammers and he had to talk with our ISP to remove us from the spammers list.

My question is: How to prevent getting marked as spammer ?

I tried to google this, but all I find is how to prevent getting spam e-mails, but nothing about sending e-mails.